The mid-Piacenzian Warm Period (mPWP; 3264–3025 ka) represents the most
recent interval in Earth's history where atmospheric CO2 levels were
similar to today. The reconstruction of sea surface temperatures (SSTs) and
climate modelling studies has shown that global temperatures were 2–4 ∘C warmer than present. However, detailed reconstructions of
marginal seas and/or coastal zones, linking the coastal and continental
climate evolution, are lacking. This is in part due to the absence of
precise age models for coastal sedimentary successions, as they are
generally formed by dynamic depositional systems with varying sediment and
freshwater inputs. Here, we present a multi-proxy record of Pliocene climate
change in the coastal southern North Sea basin (SNSB) based on the
sedimentary record from borehole Hank, the Netherlands. The marginal marine
setting of the Hank borehole during the late Pliocene provides an excellent
opportunity to correlate marine and terrestrial signals due to continental
sediment input mainly derived from the proto-Rhine–Meuse River. We improve
the existing low-resolution palynology-based age model for the Hank borehole
using stable oxygen and carbon isotope (δ18O and
δ13C) measurements of the endobenthic foraminifera species Cassidulina laevigata, integrated
with biochrono- and seismostratigraphy. Identification of hiatuses and
freshwater effects in the record allows us to isolate glacial–interglacial
climate signals in order to tune the endobenthic oxygen stable isotope record to a
global benthic δ18O stack. This results in a tuned age
framework for the SNSB for the late Pliocene (∼3190–2770 ka). Our multi-proxy climate reconstruction for the interval which covers
part of the mPWP (∼3190–3000 ka) shows a strong agreement
between lipid biomarker and palynology-based terrestrial temperature
proxies, which suggest a stable climate, 1–2 ∘C warmer than
present. In the marine realm, however, biomarker-based SSTs show a large
range of variation (10 ∘C). Nevertheless, the fluctuation is
comparable to other SST records from the North Atlantic and Nordic Seas,
suggesting that a common factor, possibly ocean circulation, exerted a
strong influence over SSTs in the North Atlantic and the North Sea at this
time.
Introduction
The Pliocene Epoch (5.33–2.6 Ma) is a frequently targeted interval for
palaeoenvironmental reconstructions because it is considered an analogue for
future climate change. For example, atmospheric CO2 concentrations
(380–420 ppmv; Seki et al., 2010; Zhang et al., 2013) and continental
configurations during the Pliocene were largely similar to present. Detailed
proxy and model comparisons for the so-called mid-Piacenzian Warm Period
(mPWP; 3264–3025 ka) have been the focus of the Pliocene Research,
Interpretation and Synoptic Mapping (PRISM) group (Dowsett et al., 2010,
2013) and reveal that global temperatures were on average 2–4 ∘C warmer than present (Haywood et al., 2013). This makes the mPWP an
excellent interval to investigate a warmer world associated with the
scenarios for our (near) future summarized by the Intergovernmental Panel on
Climate Change (IPCC, 2014).
Our understanding of Pliocene climate is largely based on sea surface
temperature (SST) reconstructions (e.g. Dowsett et al., 2012), which
indicate that global SSTs were 2–6 ∘C warmer than present.
Relatively fewer temperature records exist for the terrestrial realm
(Zagwijn, 1992; Salzmann et al., 2013). These records also indicate climate
was warmer than present; however, these temperatures are less well
constrained due to potential confounding influence of humidity on the
temperature reconstructions (Guiot, 1990) and the poorer age control on
terrestrial sediment sequences. There are even fewer studies that examine
the phase relations and amplitude of variability in coupled land–sea changes
(e.g. Kuhlmann et al., 2006), although this information is of key interest
for understanding heat transport and the hydrological cycle during the
Pliocene. Sediments on continental shelves receive inputs from both the
terrestrial and marine environment and would thus contain an archive of
land–sea climate evolution. The North Sea basin shelf is a site that
potentially hosts a combined record of SST evolution and climate change in
the north-western (NW) European continent during the Pliocene due to input
of terrestrial material by large European rivers and the active subsidence
that provides sediment accumulation space (Gibbard, 1988). Moreover,
significant warming of its waters since the second half of the 20th
century (0.6 ∘C rise on average in the period 1962–2001; Perry
et al., 2005) indicates the sensitivity of the area for recording climate
change. The region has been a type area for Pliocene and early Pleistocene
terrestrial stages (see overview in Zagwijn, 1992), but most studied
sections lack absolute dating and land–sea correlation, as they target
fragmentary fluvial successions (Donders et al., 2007; Kemna and Westerhoff,
2007). However, the shallow marine deposits of the southern North Sea basin
(SNSB) allow better chronostratigraphy building through integrated
palaeomagnetic, isotope, and biostratigraphic approaches (e.g. Kuhlmann et
al., 2006; Noorbergen et al., 2015; Donders et al., 2018).
Despite the promising preconditions that should enable Pliocene climate
reconstruction using the sedimentary archive of the SNSB, the generation of
an independently calibrated age model for coastal zone sediments is often
complicated by complex interactions between sea level, sediment supply, and
biotic factors (e.g. Krantz, 1991; Jacobs, 2008; Noorbergen et al., 2015;
Donders et al., 2018), which may alter sedimentation rates or cause hiatuses
resulting from periods with erosion or non-deposition. Indeed, the Pliocene
SNSB was a dynamic system in which multiple westward advances of the
Eridanos and Rhine–Meuse rivers generated clinoform successions (Jansen et
al., 2004; Kuhlmann and Wong, 2008; Harding, 2015). The sedimentary record
thus needs to be critically evaluated on its stratigraphic continuity before
it can be compared with records from adjacent areas, such as the Nordic Seas
and the North Atlantic. Munsterman (2016) reported a Pliocene-age sequence
of coastal marine sediments from Hank, located in the south-west of the
Netherlands. The current age framework for the sequence is based on first
and last occurrence dates (FODs and LODs, respectively) of dinoflagellate cysts (Dearing
Crampton-Flood et al., 2018). Due to the lack of an independent age
constraint in the SNSB, FODs and LODs were inferred from those in the Nordic
Seas and the North Atlantic, introducing an unknown range of age uncertainty
to the biostratigraphic age model (Dearing Crampton-Flood et al., 2018).
Furthermore, the resolution of the age model is too low (nine biostratigraphic
age tie points for the interval ∼4.5–2.5 Ma) to identify
possible hiatuses or changes in deposition, preventing comparison of the
record to other archives from the Northern Hemisphere.
The established method for age model construction involves measuring the
stable oxygen isotope content (δ18O) of foraminiferal tests and
matching the variability to a global benthic δ18O reference
stack, such as LR04 (Lisiecki and Raymo, 2005). However, in coastal settings
this method is complicated due to isotopically lighter freshwater input,
which alters the δ18O value of the foraminiferal tests
(Delaygue et al., 2001; Lubinski et al., 2001). Recently, Noorbergen et al. (2015) were successful in creating a tuned age model for the early
Quaternary shallow marine interval of borehole Noordwijk, also located in
the SNSB, using the δ18O and δ13C values of the
endobenthic foraminifera (Bulimina aculeata, Cassidulina laevigata, and Elphidiella hannai). The depth habitat of endobenthic
foraminifera in the sediment provides a moderate degree of shelter from
disturbances such as reworking by bottom currents and freshwater input.
Although vital and microhabitat effects still influenced the absolute
δ18O values of these foraminifera and caused an offset towards
more positive values, the trends in δ18O at Noordwijk clearly
resembled those of LR04 (Lisiecki and Raymo, 2005; Noorbergen et al., 2015).
In this study, we follow the approach of Noorbergen et al. (2015) and
establish δ18O and δ13C records measured from the
endobenthic foraminifera Cassidulina laevigata in the Hank borehole to improve the current
low-resolution biostratigraphic age model (Dearing Crampton-Flood et al.,
2018) for the Pliocene SNSB. Reconstruction of the age model is further
supported by the identification of hiatuses based on seismic information and
gamma ray logging. Subsequently, we complement the existing mean annual
temperature (MAT) record for Pliocene NW Europe based on soil bacterial
membrane lipid distributions stored in the Hank sediments (Dearing
Crampton-Flood et al., 2018), with multi-proxy records of SST, prevailing
vegetation, and terrestrial input based on lipid biomarker proxies, pollen,
and dinoflagellate cysts. This enables us to for the first time directly
compare marine and terrestrial climate evolution of the SNSB and continental
NW Europe during the mid-Piacenzian Warm Period.
MethodsGeological setting and study site
The Pliocene North Sea was confined by several landmasses except towards
the north, where it opened into the Atlantic (Ziegler, 1990). At times,
there may have also been a connection via the English Channel to the North
Atlantic, indicated by planktonic elements and bryozoan-dominated deposits
in the Pliocene Coralline Crag Formation in eastern England (Fig. 1; Funnel,
1996) that bear resemblance to modern deposits. However, the connection of
the North Sea to the North Atlantic via the English Channel may only have
existed during periods of high sea level (Gibbard and Lewin, 2016). Modern
modelled transport estimates from the Hybrid Coordinate Ocean Model (HYCOM)
indicate that the total mean inflow at the northern boundary is
approximately 14 times higher than that of the English Channel (Winther and
Johannessen, 2006). Thus, water inflow/outflow through the English Channel
was probably limited, regardless of whether there was an opening to the
North Atlantic or not. In addition to a main marine water supply via the
North Atlantic, the Eridanos River, draining the Fennoscandian shield, and
the proto-Rhine–Meuse River, draining north-western Europe, delivered
freshwater to the North Sea in the Pliocene (Fig. 1). The proto-Rhine–Meuse
river system existed for a large part of the Pliocene, initially draining
the Rhenish Massif and later making a connection with the Alps in the latest
Pliocene (Boenigk, 2002). During the Pliocene, the sediment supply by the
Eridanos river system to the Ruhr Valley rift system was limited, such that
the Rhine–Meuse river system was the predominant supplier of sediments in
the study area (Westerhoff, 2009). The water depth of the North Sea during
the Pliocene and the Pleistocene was approximately 100–300 m in the central
part of the basin (Donders et al., 2018) and approximately 60–100 m in the
southern North Sea basin (Overeem et al., 2001).
Pliocene palaeogeography in the North Sea basin (Gibbard and Lewin,
2003; Knox et al., 2010). The location of the Hank borehole is represented
by a red star. Major rivers and sediment inputs are represented by blue and
orange arrows, respectively. Other locations mentioned in the text are
indicated. Figure modified from Gibbard and Lewin (2016).
The study site (51∘43′ N, 4∘55′ E) is
located within the current Rhine–Meuse–Scheldt delta in the region around
Hank, the Netherlands. The Hank site is located within the Ruhr Valley Rift,
a region that experienced relatively high tectonic subsidence during the
late Cenozoic (Van Balen et al., 2000). The current drainage area of the
Rhine–Meuse–Scheldt river system is 221 000 km2; however it was likely
smaller in the Pliocene (van den Brink et al., 1992; Boenigk, 2002). In
2001, air-lifting well technology was used to drill the Hank borehole
(B44E0146) to a base of 404 m. Intervals were drilled every 1 m such that
each sample taken from the metre intervals is an integrated mixture. The
gamma ray log of the borehole is readily accessible from an online database
(https://www.dinoloket.nl, last access: 15 April 2019). In addition, a seismic section is available and covers an
east–west transect of the Meuse River (Maas2002 survey, https://www.nlog.nl, last access: 10 February 2019). The
lithology of the Hank borehole (Fig. 2a) is described by the Geological
Survey of the Netherlands (TNO) and Dearing Crampton-Flood et al. (2018). In
short, the base of the succession corresponds to the upper part of the
shallow marine Breda Formation, followed by the sandy, occasionally silty,
and clay-rich marine delta front deposits sometimes containing shell
fragments, so-called “crags”, belonging to the Oosterhout Formation. The
overlying Maassluis Formation contains silty shell-bearing deltaic-to-estuarine deposits. For this study, the interval 404–136 m was considered,
covering ∼4.5–2.5 Myr based on the biostratigraphic age
model of Dearing Crampton-Flood et al. (2018).
Marine proxies for the Hank borehole. (a) The depth and lithology of
the Hank sediments, with shell material qualitatively indicated by shell
symbols. (b) The smoothed gamma ray (GR) log (https://www.dinoloket.nl, last access: 15 April 2019). (c) Stable
oxygen and (d) stable carbon isotope records for the endobenthic
Cassidulina laevigata. (e) SST records based on TEX86 (blue diamonds), U37K′ (red triangles), and LDI (green squares). (f) Percent cold taxa of
dinoflagellate cysts. Intervals 1, 2, and 3 discussed in the text are
indicated by green (early Pliocene), grey (mid-Pliocene), and purple (late
Pliocene–early Pleistocene). The tuned interval and the position of the
hiatus marking the M2 glacial event are represented by a black line.
Stable isotopes
Deep sea δ18O and δ13C records generally oscillate
in antiphase during the Quaternary, due to the waxing and waning of large
ice caps on the Northern Hemisphere (e.g. Ruddiman, 2001). During glacial
periods, benthic foraminifera incorporate relatively more δ18O in their calcite test, since more 16O has been stored
in the ice sheets and bottom water temperatures are cold. At the same time,
they also incorporate more 12C because the total amount of vegetation
on land has been reduced during glacial periods, causing an enrichment of the
dissolved inorganic carbon (DIC) pool of the oceans. Besides, a reduced
thermohaline circulation (THC) during glacial periods may have reduced the
contribution of 12C-depleted North Atlantic Deep Water to the deep
ocean with respect to the 12C-enriched Antarctic Bottom Water
component. Hence, for the intervals in which the trends in δ18O and
δ13C records of the Hank borehole move in antiphase, we
interpreted these changes as being the result of glacial–interglacial
variability. However, when the δ18O and δ13C are
not inversely related, i.e. show a positive correlation, other factors such
as riverine freshwater inflow, reworking, and diagenetic influences were
most likely the dominant control.
Sediment samples (n=269) from the interval 404–136 m were washed and
passed over a series of sieves, after which the > 125
and > 63 µm fractions were collected and dried at 40 ∘C. Well-preserved (i.e. shiny tests) foraminifera of the
endobenthic species Cassidulina laevigata of around the same size were picked from the
> 125 µm fraction. Due to the scarcity of foraminifera in
some samples, tests were left uncrushed in order to conserve enough material
for isotope analysis. The foraminifera were washed ultrasonically in water
before weighing. Between 10 and 60 µg of intact tests were weighed
per sample. The δ18O and δ13C values were measured
on a Thermo Scientific Gas Bench II (Thermo Fisher Scientific) connected to a Delta V
mass spectrometer. An in-house NAXOS standard and an internationally
accepted NBS-19 standard (δ18O=-2.20 ‰;
δ13C=1.95 ‰) were used to calibrate
measured isotope ratios to the Vienna Pee Dee Belemnite (VPDB) standard.
Palynology
Organic-walled dinoflagellates that form a cyst during their life cycle are
referred to as dinocysts, and they are preserved in sediments (Head, 1996).
Dinocyst assemblages in marine sediments are investigated to infer
environmental parameters such as temperature and productivity in surface
waters (Rochon et al., 1999; Zonneveld et al., 2013) and can be used as
such to reconstruct past climate changes in downcore sediment records (e.g.
Pross and Brinkhuis, 2005; Hennissen et al., 2014, 2017). Terrestrial
palynomorphs are derived from vegetation and are delivered to coastal marine
sediments through wind or river runoff. The pollen and spore (or sporomorph)
assemblage in a downcore sediment record like the coastal marine Hank site
can thus indicate the type of vegetation in the nearby continent, which can
then be used to infer precipitation and/or temperature regimes of the source
area (e.g. Heusser and Shackleton, 1979; Donders et al., 2009; Kotthoff et
al., 2014).
Standard palynological techniques were used to process 82 selected samples.
HCl and HF digestion followed by 15 µm sieving was carried out
according to Janssen and Dammers (2008). Both dinocysts and sporomorphs were
counted under a light microscope at 400× magnification until a minimum of
200 specimens were found. Rare species were identified during a final scan of
the microscope slide. For dinocysts, the taxonomy of Williams et al. (2017)
is used.
Some dinocyst taxa prefer cooler (sub)polar waters; hence we take the sum of
those taxa and use that as an indicator for SST in the SNSB. We calculate
the percent of cold-adapted dinocysts as the sum of the following species, including Bitectatodinium spp., Habibacysta tectata, Filisphaera filifera, Headinium spp., Filisphaera spp., Islandinium spp., Habibacysta spp., Islandinium euaxum, and
Bitectatonium tepikiense, over the sum
of all dinocysts in the Hank borehole, following the approach adopted by Versteegh and Zonneveld (1994), Donders
et al. (2009, 2018), and De Schepper et al. (2011).
A subset of 25 samples was analysed for detailed pollen assemblages to
provide independent long-term trends in climate and vegetation cover. Late
Neogene pollen types can, in most cases, be related to extant genera and
families (e.g. Donders et al., 2009; Larsson et al., 2011). Percent
abundances are calculated based on total pollen and spores excluding
bisaccate taxa, freshwater algae, and Osmunda spores due to peak abundance in one
sample of the latter. Bisaccate pollen abundances are excluded because they
are heavily influenced by on- to offshore trends (Mudie and McCarthy, 1994)
and therefore do not primarily represent tree abundance.
The terrestrial / marine (T / M) ratio of palynomorphs takes the sum of
all sporomorphs and divides by the sum of all sporomorphs and dinocysts, T/(T+M). The
sum of sporomorphs excludes bisaccate taxa. The T/M ratio is commonly used
as a relative measure of sea level variations and therefore distance to the
coast (e.g. Donders et al., 2009; Kotthoff et al., 2014).
Lipid biomarkers and proxies
We use three independent organic temperature proxies for sea surface
temperature based on different lipid biomarkers. The TEX86 is a proxy
based on the temperature sensitivity of isoprenoidal glycerol dialkyl
glycerol tetraethers (isoGDGTs), membrane lipids of marine archaea (Schouten
et al., 2002). An increase in the relative abundance of isoGDGTs containing
more cyclopentane moieties was found to correlate with SSTs (Schouten et
al., 2002). Here we use the global core-top calibration of Kim et al. (2010)
to translate TEX86 values to SSTs. Since isoGDGTs are also produced in
soils, albeit in minor amounts, they may alter the marine temperature signal
during periods with large contributions from land. The relative input of
(fluvially discharged) terrestrial organic matter (OM) can be determined
using the ratio of branched GDGTs (brGDGTs), which are produced in soils
(Sinninghe Damsté et al., 2000; Weijers et al., 2007) and rivers (Zell
et al., 2013), with crenarchaeol, an isoGDGT exclusively produced by marine
Thaumarchaeota (Sinninghe Damsté et al., 2002). This ratio is quantified
in the Branched And Isoprenoid Tetraether (BIT) index (Hopmans et al.,
2004), where high BIT indicates a high continental OM input and vice versa.
A BIT index > 0.3 is generally used as a cutoff for the validity
of TEX86-based SST estimates (Weijers et al., 2006). Secondly, the
U37K′ index is used as a proxy for SST based on the degree
of unsaturation of C37 alkenones produced by marine haptophyte algae
(Prahl and Wakeham, 1987). An increased abundance of the tri-unsaturated relative to
the di-unsaturated C37 alkenones, expressed as the U37K′ index, is linked with decreasing temperature, an adaptation
thought to retain membrane fluidity in cooler environments (Marlowe et al.,
1984). U37K′ values can be converted to SSTs using the
global core-top calibration of Müller et al. (1998), with a calibration
error of 1.5 ∘C. Finally, SSTs can be reconstructed based on the
relative distribution of long-chain diols, which are dihydroxylated lipids
with 22–38 carbon atoms. The C28 1,13-, C30 1,13-, and C30
1,15-diols are most commonly found in seawater and have a putative
phytoplankton source (Volkman et al., 1992; Rampen et al., 2007, 2011). The distribution of these three diols is used to formulate the
long-chain diol index (LDI), which can be converted to SST using the
calibration of Rampen et al. (2012), with a calibration error of 2.0 ∘C. Furthermore, since freshwater eustigmatophyte algae produce
C32 diols (Volkman et al., 1992, 1999; Gelin et al., 1997), the
percentage of the C32 diol versus that of the marine C28 1,13-,
C30 1,13-, and C30 1,15-diols can be used as an indicator for
freshwater discharge (Lattaud et al., 2017).
Lipid biomarkers were previously extracted from the sediments (n=155)
and separated into polarity fractions (Dearing Crampton-Flood et al., 2018).
The polar fractions, containing GDGTs, were analysed on an Agilent 1260
Infinity ultra-high performance liquid chromatography instrument (UHPLC)
coupled to an Agilent 6130 single quadrapole mass detector with settings
following Hopmans et al. (2016). For more details on method and solvent
programme see Dearing Crampton-Flood et al. (2018). Selected ion monitoring
(SIM) was used to detect [M-H]+ ions of the isoGDGTS: m/z 1302, 1300,
1298, 1296, 1292.
After GDGT analysis, polar fractions were silylated by the addition of
N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) and pyridine (60 ∘C; 20 min). A Thermo Scientific TRACE gas chromatograph (GC) coupled to a
Thermo Scientific DSQ mass spectrometer (MS) was used to analyse long-chain diol
distributions in SIM mode (m/z 299, 313, 327, and 341) at the NIOZ Royal Netherlands Institute. The
temperature programme was as follows: 70 ∘C for 1 min, then ramped to 130 ∘C at 20 ∘C min-1, then ramped to 320 ∘C at 4 ∘C min-1, and then held for 25 min.
Ketone fractions, containing the C37 alkenones, were analysed using gas
chromatography with flame ionization detection (GC-FID). Samples were
injected (1 µL) manually on a Hewlett Packard 6890 series GC system
equipped with a CP-Sil 5 fused silica capillary column (25m×0.32mm; film
thickness 0.12 µm) and a 0.53 mm precolumn. The oven temperature
programme was similar to that used for long-chain diol analysis.
ResultsStable isotopes of Cassidulina laevigata
Foraminifera preservation in the intervals 404–386 and 204–136 m was
either very low or nonexistent. Furthermore, the low abundance and poor
preservation of foraminifera in the crag material (220–205 m in Fig. 2c) also
presented a challenge for picking. The δ18Ocass. values of
foraminifera in the sediment from the remaining intervals (n= 136) range
from -1.0 ‰ to 3.6 ‰ (Fig. 2c). The variability in δ18Ocass. values between maxima and adjacent minima in the Hank
record ranges from ∼1 ‰ to 4 ‰. The stable
carbon isotope record varies between -3.8 ‰ to 0.6 ‰ (Fig. 2d), and the variability is ∼0.3 ‰ to 2.3 ‰.
Discounting the sample at 206 m, the variability in the δ13Ccass. record is approximately ∼1‰ (Fig. 2d). The δ18Ocass. and δ13Ccass. records show antiphase relationships in the intervals
378–360, 345–310, 304–292, 282–279, 275–270, 252–242, 232–227, and
224–218 m (Fig. S1a in the Supplement).
Seismic profile
The ∼15 km east to west seismic profile of the Meuse River,
including the location of the Hank borehole, spans a depth of > 500 m (Maas2002 survey, https://www.nlog.nl, last access: 10 February 2019; Fig. 3). Comparison of the formations of
the Hank borehole with the seismic depth profile in Fig. 4 indicates that
the Breda Formation at 404–370 m is characterized by horizontal-reflection
patterns, likely indicating shallow marine conditions. The eastern
continuation of the seismic line reveals that these horizontal strata can be
interpreted as shelf toesets of westward-prograding deltaic clinoforms.
Seismic east to west depth profile at the Meuse River (from Maas2002
survey, https://www.nlog.nl, last access: 10 February 2019) with the location of the Hank borehole (B44E0146) and
corresponding formations indicated. The smoothed gamma ray log (from
https://www.dinoloket.nl, last access: 15 April 2019; white) and lithology of the borehole are provided for
context. Stars and age ranges refer to the biostratigraphic age model of
Dearing Crampton-Flood et al. (2018). The orange and yellow lines represent
the boundaries of the Breda and Oosterhout (revised) and the Oosterhout and
Maassluis formations, respectively.
Terrestrial proxies for the Hank borehole. (a) The depth and
lithology of the Hank sediments, with shell material qualitatively indicated
by shell symbols. (b) The smoothed gamma ray (GR) log (from https://www.dinoloket.nl, last access: 15 April 2019) and
(c) the relative input of terrestrial organic material to the Hank sediments
based on the terrestrial / marine ratio of palynomorphs (black line), the
Branched and Isoprenoid Tetraether (BIT) index (orange circles), and the
percent of C32 diol (blue squares). Pollen records expressed as percent of total
pollen: (d) other conifers (brown line), Taxodioidae (orange line) and MAT
(in degrees Celsius; green line; Dearing Crampton-Flood et al., 2018), (e) angiosperm trees, (f) herbs, (g) heather, (h) spores, (i)Pinus, and (j)Osmunda. Intervals 1, 2, and 3 discussed in the text are indicated by green
(early Pliocene), grey (mid-Pliocene), and purple (late Pliocene–early
Pleistocene). The tuned interval and the position of the hiatus marking the
M2 glacial event are represented by a black line.
Palynology
The palynomorphs in the Hank sediments are well preserved. The borehole can
be divided into three main intervals according to the (co)dominance of the
marine/terrestrial palynomorphs: 1, 2, and 3, which roughly correspond to the (1) early Pliocene, (2) mid-Pliocene, and (3) late Pliocene–early Pleistocene. In the deepest part of the borehole (404–330 m; interval 1), the marine
component of the palynomorph assemblage clearly exceeds the terrestrial, as
evidenced by the low T/M values (Fig. 4c). An isolated sporomorph peak and
(sub)polar dinocyst peak are visible at 383 m (Figs. 2f, 4c). Interval 2 from
330 to 187 m shows a fluctuating ratio between the marine and terrestrial
elements (Fig. 4c). The cold-adapted dinocysts also show fluctuations
indicating alternating warmer and cooler periods (Fig. 2f). One striking
feature is the increase in cold-adapted dinocyst abundance and simultaneous
Osmunda acme at 305 m (Figs. 2f, 4j). The third, interval 3, spans the upper part of
the borehole (187–136 m), and sporomorphs in particular dominate this
interval, visible by the consistently high T/M (Fig. 4c). Interval 3 shows
an increased occurrence of coastal marine genera, like Lingulodinium. The increased gamma
ray values at ∼175 m (Fig. 2b) are the result of the abundance
of (shell) concretions, not clays, and as such, do not indicate a more
distal environment but rather a development toward a more proximal
environment. At 154–153 m, the marine indicators in the borehole are
reduced to just 0.5 % of the total sum of palynomorphs. However, the
highest abundance of cold-adapted dinocysts, mostly composed of taxa like
Habibacysta tectata, is at 154 m (Fig. 2f). This depth is also marked by the complete
disappearance of the dinocyst genus Barssidinium spp. with an LOD at 157 m (Dearing
Crampton-Flood et al., 2018). (Sub)tropical species like Lingulodinium machaeophorum, Operculodinium israelianum, Spiniferites mirabilis,
Tectatodinium pellitum, and Tuberculodinium vancampoae are missing at this depth. The uppermost (154–136 m) interval
indicates an estuarine to deltaic environment, due to the presence of
freshwater and brackish water algae species Pediastrum and Botryococcus. In contrast, the
freshwater indicators are (almost) absent in intervals 1 and 2. The
assemblages of interval 3 are also characterized by a fluctuating abundance
of cold-adapted dinocysts (Fig. 2f).
The pollen assemblages are dominated by tree pollen, particularly conifers – Pinus, Picea, Abies, Taxodioidae-type (including Glyptostrobus and Taxodium), Sciatopitys, and Tsuga – but with increasing proportions of grasses
(Poaceae, Cyperaceae) and heather (Ericales) in interval 2 and
significantly increased amounts of fern spores from 260 m and up (Fig. S1).
Exclusion of the bisaccate types from the percentage sum (to counter effects
of sea level change on the diverse transport capacity of pollen; Neves
effect) causes the percentage of pollen sum to be relatively low
(∼100 grains/sample). The current pollen record should
therefore be used for main quantitative trends and not to delineate
individual ranges of taxa. The angiosperm tree abundances average about
20 % and shows no significant long-term change towards the top of the
sequence (Fig. 4e). The angiosperm tree pollen record is diverse, although
few taxa are continuously present and consists mostly of Quercus robur-type with
significant proportions of Pterocarya, Fagus, Carpinus and, above 240 m, Ulmus (Fig. S1). The
Taxodioidae-type shows a distinct long-term decline superimposed by three shorter
minima in the end of interval 2 and beginning of 3, occurring at 205, 235
and 170 m (Figs. 4d, S2).
Lipid biomarkers
IsoGDGTs are present in high abundances throughout the borehole, as
evidenced by the high total organic carbon (TOC)-normalized concentrations
of crenarchaeol (0.2–130 µg g-1 TOC; Dearing Crampton-Flood et
al., 2018). IsoGDGT-based SSTs are reconstructed for those sediments where
BIT < 0.3, i.e. between 404 and 219 m (n=66).
TEX86H-reconstructed SSTs range between 7 and 13 ∘C
but do not show a clear trend over time (Fig. 2e).
Alkenones are present in the majority (n=111) of the samples. However,
they are below the detection limit in many of the sediments from 250 to 200 m.
Alkenones re-emerge in the interval from 197 to 178 m. The U37K′ index values range between 0.30 and 0.83 and correspond to a SST
range of 8–24 ∘C (Fig. 2e). In the early Pliocene (1), the
U37K′ SST record shows the largest fluctuations in
temperature (ΔT=15∘C) and an average SST of 19 ∘C. Similar variability (ΔT=14∘C) is
observed in interval 2 (middle–late Pliocene), although the average SST
drops slightly to 17 ∘C. Alkenones are present around or below
the detection limit in interval 3 (late Pliocene–early Pleistocene), so no SSTs can
be calculated. Notably, U37K′ SSTs show a warming of 11 ∘C during the interval between 290 and 260 m (Fig. 2e).
Long-chain diols are below the detection limit in a large proportion of the
Hank borehole. SSTs can be reconstructed for a select few samples in
intervals 1 and 3, and they show scattered temperatures in a range of 13 ∘C (Fig. 2e). The sediments in interval 2 contain enough diols to
enable a semi-continuous SST reconstruction. The range of LDI SSTs in this
section is 4–18 ∘C (Fig. 2e). The record shows a strong warming
trend of ∼12∘C from 295 to 263 m, coeval with the
trend in the U37K′ record (Fig. 2). The percent of C32 diol is generally high (∼36 %–57 %) in intervals 1 and 2 (early–middle
Pliocene; Fig. 4c), indicating a modest to strong freshwater input (cf.
Lattaud et al., 2017). The percent of C32 diol slightly decreases (10%)
over 294–264 m, indicating a gradually decreasing influence of riverine OM
and/or an increase in the abundance of the marine C28 1,13-, C30 1,13-, and
C30 1,15-diols. In interval 3, the percent of C32 diol exhibits
a strong increasing (21–59%) trend (187–136 m; Fig. 4c).
DiscussionDepositional record and unconformities
The changes in depositional environment in the Hank borehole from open
marine to coastal marine and successively estuarine conditions are based
upon the BIT, TOC, and δ13Corg records presented in
Dearing Crampton-Flood et al. (2018) and the biological changes in the
abundances of typical marine (dinocysts and test linings of foraminifera),
estuarine/freshwater algae species, and sporomorph assemblages (Munsterman,
2016), summarized in the T/M ratio. A transition of marine OM during the
Pliocene to more terrestrial OM input towards the Pleistocene starts
approximately at 190 m as evidenced by these indicators (Fig. 4c). This
reflects the increasing influence of the Rhine–Meuse River to the site. This
progradation of the Rhine–Meuse River is confirmed by the westward
progradation of the depositional (delta) system that can be seen in the
seismic section (Sect. 3.2, Fig. 3).
Toward the long-term shallowing trend in the borehole, several depositional
changes and unconformable surfaces are recognized in the seismic profile,
which need to be taken into account regarding the construction of a valid
age model. The transition from the Breda Formation to the overlying
Oosterhout Formation is marked by a distinct angular unconformity referred
to as the late Miocene unconformity (LMU; Munsterman et al., 2019; Fig. 3).
The seismic data of the overlying Oosterhout Formation (middle–late Pliocene)
indicate a twofold subdivision. The lower unit of the subdivision at
370–314 m is characterized by convex-downward reflection patterns that
correspond to an open marine signature (with corresponding low sedimentation
rates). This is confirmed by the transition to finer-grained sediments
(silts) over 381–352 m attributed to a more distal setting (Fig. 2a). This
interval is also characterized by an increased abundance of dinocysts with a
preference for open marine conditions, like the genus Spiniferites. The facies of
interval 352–338 m indicates shallow to open marine conditions with a
temperate to (sub)tropical SST. A coarsening upward trend is corroborated by
the gradual decrease in gamma ray values (Fig. 2b). In the second Oosterhout
unit from 314 to 157 m the environment is shallow marine, and several stacked
clinoform sets are visible in the seismic profile (Fig. 3). The transition
between the two Oosterhout units is clearly visible as a downlap surface
around 314 m that corresponds to a sequence boundary and possible hiatus
(Fig. 3). This is coupled with a dramatic decrease in sedimentation rate in
the initial age model, which places this interval within the scope of the M2
glacial event (∼3.3 Ma; Dearing Crampton-Flood, 2018).
Above the sequence boundary at 314 m, an increase in water depth to
∼80–100 m can be deduced from the height of the clinoforms
where the topsets represent the fluvial system debouching at the coastline.
This water depth estimate is comparable to the estimate for Pliocene water
depth from an integrated seismo-stratigraphic study of the SNSB (Overeem et
al., 2001). The topset beds in the Hank borehole show an abundance of shell
crag facies material corroborating the coastal setting (Figs. 2a, 3). The
stratigraphic stacking is first purely progradational (clinoforms) and
changes to both progradational and aggradational higher up, suggesting
progressive fluvial influence replacing the marine environment. In the Hank
borehole, this change is marked by a distinct clay layer at 292–271 m (Fig. 2a). In the upper part of the second Oosterhout Formation at depths of 260 m
and upwards, the increased proportion of heather and grasses is generally
considered indicative of colder and drier terrestrial climate (Faegri et
al., 1989; Fig. 4). The decrease in Taxodioidae-type pollen over interval 2 (Fig. 4d)
and the Oosterhout to the Maassluis formations further indicates a cooling
terrestrial climate and is classically recognized as the top Pliocene (late
Reuverian) in the continental zonation of Zagwijn (1960), although in the
onshore type area the sequences are most likely fragmented time intervals
bounded by several hiatuses (Donders et al., 2007).
At the transition of the Oosterhout Formation to the Maassluis Formation,
concave-downward reflection patterns may reflect channel incisions into the
topsets of the Oosterhout Formation (Fig. 3). The Maassluis Formation (late
Pliocene–early Pleistocene; < 2.6 Ma) is composed of horizontal and
channel-like strata in the seismic profile (Fig. 3). The environment of the
Maassluis Formation becomes more fluvio-deltaic, characterized by the
decreased abundance of dinoflagellate cysts and steep rise in the number of
sporomorphs, manifested by the high T/M values (Fig. 4c). Further
warm–temperate trees in the Maassluis interval of the record such as Carya, Liquidambar, and Nyssa
disappeared in NW Europe in the earliest Pleistocene (Donders et al., 2007),
most likely slightly above the level of the top of the Hank sequence. In
contrast, land-based studies in the Dutch–German border area (cf. Donders et
al., 2007; Westerhoff, 2009) are characterized by relatively more abrupt
last occurrences of warm–temperate taxa due to the probably incomplete
preservation of the early Pleistocene sequences.
The Plio-Pleistocene transition (2.6 Ma) occurs between 200 and 154 m (Dearing
Crampton-Flood et al., 2018). This transition is accompanied by a peak in
gamma ray values at ∼175 m (Fig. 2b). However, the coastal
marine depositional setting for the Hank borehole in the interval (upper
Oosterhout Formation) during the late Pliocene/early Pleistocene above 200 m
strongly indicates that these successions are likely not continuous but
consist of successive fragments of sedimentation representing short time
windows that are bounded by hiatuses (cf. Donders et al., 2007).
Age framework for the Pliocene southern North Sea basin.
Correlations of the interglacials of the (a) LR04 stack (Lisiecki and Raymo,
2005) to the interglacials in the (b)δ18Ocass. record for
Hank. (c) Smoothed gamma ray (GR) and (d) lithology, and depth of Hank
sediments. Dotted grey lines indicate tie points based on δ18O
values, whereas the dashed line at ∼314 m is based on
correlation of sequence boundary with the Poederlee Formation (De Schepper
et al., 2009a; Louwye and De Schepper, 2010). Biostratigraphic age estimates
(Dearing Crampton-Flood et al., 2018) are shown as black diamonds.
M2 event
There is global evidence for a large sea level drawdown during the M2 glacial event, but
the estimates of the magnitude of its extent vary greatly (38–65 m; Dwyer
and Chandler, 2009; Naish and Wilson, 2009; Miller et al., 2011, 2012). However, large uncertainties in the estimation of ice volume
prohibit any meaningful estimates of sea level for the Pliocene using the
stable isotope measurements of foraminifera (Raymo et al., 2018).
The hiatus at Hank representing the M2 glacial event
is also recognized in
sequences from the Coralline Crag in the English North Sea (Williams et al.,
2009), the Poederlee and Lillo formations in the Belgian North Sea (De
Schepper et al., 2009a; Louwye and De Schepper, 2010) and possibly the
Nordic Seas (Fig. 1, Risebrobakken et al., 2016). In the Poederlee and Lillo
formations of neighbouring Belgium (located 80 km from Hank in the present
day), De Schepper et al. (2009) and Louwye and De Schepper (2010)
hypothesize that the MIS M2 is correlated with a sequence boundary PIA1 at
approximately 3.21 Ma. In addition, equivocal temperature/assemblage signals
in the Coralline Crag Formation are hypothesized to be a result of sea-level
change associated with the M2 glacial event, which would have decreased or ceased
sedimentation entirely (Williams et al., 2009). A Pliocene benthic δ18O record adjacent to the North Sea (NS) in the Nordic Seas (ODP hole 642B;
Risebrobakken et al., 2016) also does not record any strong evidence of the
M2 event, and the authors postulated that the M2 event might have occurred during
a hiatus in the borehole or may have been a less extreme event in this
region compared to other regions (Lisiecki and Raymo, 2005). Due to the M2 event
being a globally recognized event (De Schepper et al., 2014), the records
from the East of England, Belgium, and the Hank site indicate that a hiatus
likely exists over the most acute part of the glacial event in SNSB sediment
successions. Thus, the coolest interval (with the presumed lowest sea level
and a high hinterland sediment supply) of the M2 event might have not been recorded
at Hank because of erosion.
In the sediments occurring above the hiatus marked by the sequence boundary
in Fig. 3, large variability in δ18Ocass. indicates
fluctuating climate conditions that may be associated with the onset or the
recovery of the M2 event. The fluctuations match those in the records of the BIT
index and δ13C of organic matter (see Dearing
Crampton-Flood et al., 2018), which indicate a closer proximity of the coast
to the site, likely as a result of sea level change. The major peak of
Osmunda spores (outside of pollen percentage sum) after the hiatus at
∼3210 ka (306 m) could then represent a pioneer phase of
marsh vegetation related to a rapid sea level lowering. The (sub)polar
dinocyst acme and increase in Operculodinium centrocarpum (Figs. 2, 6a) at 305 m may then represent the
restoration of the location of the Hank site to a more distal marine setting
within the confinement of the Upper Rhine Graben. The sea level drop
associated with the M2 event may have decreased the inflow of Atlantic
bottom water currents originating from the northern opening of the North Sea
(Kuhlmann et al., 2006). After the M2 event, isostasy may have then
strengthened the connection to the North Atlantic (possibly also via the
English Channel), which would have allowed the inflow of relatively warmer
and saline Atlantic Water fed by the North Atlantic Current (NAC) into the
North Sea. The dinocysts of Operculodinium centrocarpum are generally used as a tracer for the NAC in
the North Atlantic (De Schepper et al., 2009b; Fig. 6a) and may tentatively
be linked to the increasing influence of the North Atlantic to the study
site after the hiatus associated with the M2 event. However, Operculodinium centrocarpum is a
cosmopolitan species, and in the modern day with a connection via the
English Channel to the North Sea, it is not commonly found (Marret and Zonneveld,
2003; Zonneveld et al., 2013). Thus, the re-emergence of Operculodinium centrocarpum over the mPWP
interval (Fig. 6) more likely reflects the restoration of marine conditions
in the shallow SNSB after the M2 event.
The acme of Osmunda spores coincides with the occurrence of dinocysts
characteristic of (sub)polar water masses at ∼3210 ka, further
indicating cold conditions (Fig. 6). In addition, the distinct decrease in
Taxodioidae-type pollen at the same time indicates that climate conditions were also
cold(er) on the continent (Fig. 6c), which is supported by low terrestrial
mean air temperatures of 6 ∘C, independently reconstructed based
on brGDGTs (Fig. 6f; Dearing Crampton-Flood et al., 2018). In contrast, all
SST reconstructions remain stable during this M2 deglaciation/recovery
period (Fig. 6d), suggesting that cold periods on land are better recorded
in the sedimentary record than those in the marine realm. Indeed,
terrestrial proxies should represent an integrated signal over longer time
and larger space (NW Europe), compared to that of the marine proxies, which
are confined to the shallow SNSB basin and potentially mostly record warm
periods (Sect. 4.3).
Climate proxy records for the southern North Sea basin for the late
Pliocene. Age tie points (stars) based on oxygen isotope stratigraphy
(black), sequence boundary correlation (red), and biostratigraphy (blue) are
indicated. The depth interval covered by the age tying points is 206–330 m.
(a) The relative abundance of Operculodinium centrocarpum expressed as a percent total of dinocysts;
(b) percent of cold dinocysts; (c) pollen abundances for Taxodioidae (orange) and Osmunda (dark
yellow) as a percentage of the pollen sum; (d) SST records based on the
TEX86, U37K′, and LDI proxies, together with estimates (dotted
lines) from oxygen isotope measurements of bivalves (Vignols et al., 2019);
(e) the relative input of terrestrial organic material to the Hank sediments
based on the Branched and Isoprenoid Tetraether (BIT) index (Dearing
Crampton-Flood et al., 2018) and the input of fresh water based on the percent of C32
diol; (f) mean air temperature based on brGDGT-palaeothermometry (Dearing
Crampton-Flood et al., 2018); and (g) the benthic oxygen isotope stack of
Lisiecki and Raymo (2005).
Glacial–interglacial variability and tuning
Based upon the age model of Dearing Crampton-Flood et al. (2018), it is
clear that the sample resolution is too low to resolve a stable isotope
tuning on Milankovitch timescales for the older succession including the
Breda and lower Oosterhout formations (404–330 m), but it is sufficient
(i.e. < 6 kyr) to resolve individual cycles, in particular above 314 m. Notwithstanding the low resolution between 390 and 314 m, the δ18Ocass. and δ13Ccass. records move in opposite
directions, suggesting that global glacial–interglacial ice volume changes
were largely influencing the open (shallow) marine environment at Hank
during that time. In the upper part of the Hank borehole (314–200 m), the
trends in δ18Ocass. and δ13Ccass. do not
show a continuous inversed relationship (Fig. S1a), indicating that the ice
volume influence is likely obscured by other factors such as riverine
freshwater inflow.
The absolute values of the oxygen isotope measurements on Cassidulina laevigata recorded in Hank
are substantially lower by approximately 1 ‰–1.5 ‰ than
the composite benthic δ18O values in the LR04 stack (Lisiecki
and Raymo, 2005), as well as those of a nearby Pliocene benthic oxygen
isotope record from the Nordic Seas (∼2 ‰–3 ‰; Risebrobakken et al., 2016). The offset in absolute
values is unlikely due to a species-dependant effect, as δ18Ocass. values in a nearby Quaternary-age core from Noordwijk
(Noorbergen et al., 2015) were comparable to the LR04 stack (Lisiecki and
Raymo, 2005). Hence, the relatively low δ18Ocass. values in
the Hank sediments likely reflect the influence of freshwater input at this
site, which is proximal to the mouth of the palaeo-Rhine–Meuse River (e.g.
Delaygue et al., 2001; Lubinski et al., 2001; Westerhoff, 2009; Fig. 1).
Similarly, low benthic and planktic δ18O values in
sediments from the Ionian Sea coinciding with sapropel deposition were
attributed to increased freshwater influence at this time (Schmiedl et al.,
1998). Furthermore, the large δ18Ocass. variability in the
Hank record (∼1 ‰–4 ‰) compared to that
in the LR04 stack record (0.2 ‰–0.7 ‰ during the
Pliocene; Lisiecki and Raymo, 2005) indicates that the δ18O
isotopic signature of shallow seawater at the Hank site is sensitive to
freshwater input. At the delta front, wave action and winnowing contribute
to the mixing of freshwater input in the relatively shallow water column,
which explains why an endobenthic species may be affected by freshwater
influence, which results in relatively lower absolute δ18Ocass. values at the Hank site. Thus, salinity changes and
sensitivity to freshwater input affect the oxygen isotopes incorporated into
Cassidulina species at the Hank site, regardless of the endobenthic habitat.
The upper (Plio-Pleistocene transition) and lower (M2 glacial) stratigraphic
boundaries identified in Sects. 4.1 and 4.2 provide a contextual framework
to construct a higher-resolution age model for the mPWP (3264–3025 ka)
using stable isotopes of Cassidulina laevigata. The open marine signature and relatively
horizontally deposited clinoform sets in the second unit of the Oosterhout
subdivision from ∼305 to 200 m (Fig. 3) represent a relatively
continuous sedimentary record that may be suitable for age model
reconstruction, keeping in mind the potential freshwater influence on the
δ18Ocass. record.
In order to use the δ18Ocass. record for tuning purposes,
an understanding of the North Sea hydrogeography and circulation patterns
during the Pliocene must be taken into consideration. During cold periods,
the North Sea circulation slows due to the reduced sea level and inflow of
Atlantic water (Kuhlmann et al., 2006). Stratification in the North Sea due
to freshwater input from rivers combined with the sluggish circulation and
weak influence of the Atlantic waters make cooler periods problematic to
tune to, due to a δ18O signature that is probably highly
localized and erratic. Moreover, Donders et al. (2007) noted that the
coldest phase of glacials of the Plio-Pleistocene climate development of
coastal areas in the NS is likely to be marked by substantial hiatuses
caused by non-deposition and erosion, which may also preclude the use of the
transition between the warmer and cooler periods as a tuning anchor. During
warmer periods, an increased freshwater input from river outflows is also
expected, due to the supposedly wetter climate conditions during
interglacials. However, Kuhlmann et al. (2006) linked warmer periods in the
Pliocene in the central section of the southern North Sea with the
occurrence of Cassidulina laevigata, whose habitat in the modern North Sea is located in the
northern part with a strong connection to the Atlantic (Murray, 1991). Thus,
tuning the warmer periods in the δ18Ocass. record at the
Hank site with warm periods in the LR04 benthic stack is preferable due to
the strong(er) connection to the Atlantic (Kuhlman, 2004), resulting in a
relatively more regional signature of the δ18Ocass. values (Kuhlmann et al., 2006). Moreover, the chance of
disturbance/hiatuses that affect the continuity of the sediment record at
Hank is decreased in warmer periods.
Using the above reasoning, the sample with the lowest δ18Ocass. value in each cycle between ∼314 and 200 m
in the Hank record can be tuned to the lowest δ18O value
between the M2 event associated with the hiatus at 314 m (∼3.3 Ma) and the Plio-Pleistocene boundary (Sects. 4.1, 4.2) in the LR04
stack, assuming that the low δ18O values in the Hank
borehole represent the warmest part of each interglacial period (Fig. 5). Further
investigation into the variation in the δ18Ocass. cycles
in the Hank borehole isotope record reveals unique sawtooth structures,
differing from the more symmetrical pattern of cyclicity that is seen in the
Pleistocene interval of the LR04 stack. Specifically, cycles G19, G17, and
G15 display these reversed sawtooth patterns in the global benthic stack
and help pinpoint corresponding cycles in the Hank borehole record (Fig. 5).
The reconstructed time window spans ∼3190–2770 ka and thus
most of the mPWP. Based on the tuned oxygen isotope age model, the LOD of
Invertocysta lacrymosa and Operculodinium? eirikianum in the SNSB can be constrained to ∼3040 and < 2768 ka, respectively (see Dearing Crampton-Flood et al., 2018).
Late Pliocene climate reconstructionMarine proxy interpretation
Despite the fact that all three lipid biomarker proxies (TEX86,
U37K′, and LDI) are calibrated to SST, the records that
they generate show remarkable differences and are offset in temperature
(Fig. 2e). The Pliocene TEX86H SSTs are 10 ∘C on
average, which is the same temperature as the modern mean SST of the North
Sea (Locarnini et al., 2013) and contrasts with other North Sea Pliocene
temperature estimates based on ostracod, mollusc, foraminiferal, and
dinocyst assemblages (Wood et al., 1993; Kuhlman et al., 2006; Johnson et
al., 2009; Williams et al., 2009), all suggesting that the SST of the North
Sea was 2–4 ∘C warmer than present at that time. However,
present-day TEX86 reconstructions for core-top sediments in the North
Sea range between 4.1 and 9.1 ∘C (Kim et al., 2010) and thus
underestimate the observed modern SST. Lower-than-expected TEX86 values
found elsewhere have been explained by a contribution of isoGDGTs produced
by a subsurface community (Huguet et al., 2007), but given the shallow water
depth (80–100 m) of the SNSB in the Pliocene (Hodgson and Funnel, 1987;
Long and Zalasiewicz, 2011; Overeem et al., 2001; this study), it seems
unlikely that such a community would have played a role here. This can be
further confirmed by calculating the ratio of isoGDGT-2 / isoGDGT-3 ([2]/[3];
Taylor et al., 2013), whose value increases with increasing isoGDGT input
from subsurface-dwelling archaea. The [2]/[3] ratio in the Hank borehole is
2.1 on average and always well below the value associated with a deep-water
archaea community overprint (> 5; Taylor et al., 2013). Instead,
the low TEX86H SSTs are likely a result of seasonal production of
isoGDGTs. In the modern North Sea, the main period of thaumarchaeotal blooms
and associated isoGDGT production are in the winter months where ammonia is
available and competition with phytoplankton is minimal (Herfort et al.,
2006; Pitcher et al., 2011), which likely introduces a cold bias in
TEX86-based SST estimates for the SNSB. If the TEX86-derived SSTs
are interpreted as a winter signal as we argue, then the Hank Pliocene SSTs
are approximately 3–6 ∘C warmer than modern winter SSTs (van
Aken, 2008).
Conversely, U37K′-reconstructed SSTs are 16 ∘C on average and thus 2–4 ∘C higher than the temperature
estimates based on ostracod, mollusc, and foraminiferal assemblages (Wood et
al., 1993; Kuhlmann et al., 2006; Johnson et al., 2009; Williams et al.,
2009) and ca. 6 ∘C higher than modern annual mean SST. These
higher-than-expected U37K′ SSTs could in part be caused
by a species effect as a result of a contribution from alkenones produced by
freshwater haptophyte algae that have little to no correlation of
U37K′ with temperature (Theroux et al., 2010; Toney et
al., 2010). Moreover, the influence of freshwater input on salinity may
alter the main alkenone-producing communities in coastal regions (Fujine et
al., 2006; Harada et al., 2008) and thus affect the reliability of SST
estimates based on the open ocean calibration specifically adapted for Group
III alkenone producers (e.g. Emiliania huxleyi). Indeed, strong temperature fluctuations of
10 ∘C in a Holocene U37K′ record from the Sea
of Okhotsk were linked to periods with low sea surface salinity, which were
in turn correlated to high U37K′-derived SSTs (Harada et
al., 2008). In contrast, a recent study showed that alkenone producers in
particulate organic matter (POM) in a coastal bay in Rhode Island were
unaffected by a lower salinity, further illustrated by the excellent match
of the 300-year U37K′ SST record with instrumental
temperature records, despite the proximity to the river (Salacup et al.,
2019). Further, Blanz et al. (2005) showed that sediment samples from a
salinity transect covering the Baltic Sea to the North Sea showed no
relationship between U37K′ and SST in the Baltic
Proper. Only in the transition zone at Skagerrak, the SSTs were within 1 ∘C of the global calibration of Müller et al. (1998).
Although the high variability in the U37K′ SST record
and the higher-than-expected reconstructed temperatures at Hank fit with a
freshwater input as observed in the Sea of Okhotsk, low BIT index values and
T/M ratios in the Hank borehole (Fig. 4) suggest that the organic matter has
a primarily marine origin. In addition, the absence of the C37:4 alkenone in the Hank sediments, a biomarker tentatively linked with
coastal or freshwater haptophytes (Cacho et al., 1999), suggests that the
U37K′ should mostly represent SSTs, although studies
from the Baltic Sea indicate that the relative contribution of C37:4 alkenones only increases at salinities lower than 8 psu (Schulz et al.,
2000; Kaiser et al., 2017). Thus, the moderate relation between the
percent of C32 diol and U37K′ derived SST for the tuned
interval (n=26; R2=0.32), suggests that freshwater input may
at times have influenced the U37K′ SSTs.
Alternatively, the higher U37K′ SSTs can be a result of
increased production in the spring or summer (Chapman et al., 1996;
Rodrigo-Gámiz et al., 2014). Indeed, summer temperatures in the
Oosterhout Formation (Ouwerkerk, Netherlands) and contemporaneous Lillo
Formation in Belgium (Valentine et al., 2011) recorded from benthic bivalves
range from 14.9 to 20.4 ∘C, which is similar to the range of
U37K′ SSTs in Fig. 2e. This would mean that summer SSTs
were high and very variable during the Pliocene. Although quite variable in
the earlier (∼3250–3150 ka) part of the record,
U37K′ SSTs warmed by approximately 10 ∘C over
the latter part of the tuned interval from 3150 to 3050 ka (Fig. 6d).
Reconstructed modern SSTs from surface sediments in Skagerrak region near
the opening to the Baltic Sea range from 10 to 12 ∘C,
slightly higher than observed annual SSTs and resembling those of May to June
more (Blanz et al., 2005). Thus, there is evidence in the modern North Sea
adjacent area for the U37K′ recording summer
temperatures (coinciding with haptophyte blooms).
LDI SSTs are at first 2 ∘C cooler than the TEX86 record and
then increase toward the same absolute SSTs as in the U37K′ record (Figs. 2, 6d). Large discrepancies of 9 ∘C
between TEX86 and LDI-derived SSTs have been observed in the Quaternary
of south-eastern Australia (Lopes dos Santos et al., 2013), which the
authors attributed to seasonal production of isoGDGTs in the cooler months
and long-chain diols in the warmer months. In late Pliocene sediments from
the central Mediterranean, LDI SST estimates were slightly lower than
U37K′ SSTs; however this was within the error range of the
proxies (Plancq et al., 2015). Due to the recent advent of the LDI proxy
and the scarcity of other multi-proxy studies (De Bar et al., 2018; Lattaud
et al., 2018) comparing the LDI to U37K′ and TEX86
SSTs in the same sediment samples, further discussion on this topic is
limited.
mPWP climate
The mPWP is almost entirely covered by the oxygen isotope age-tuned interval
of the Hank record, which starts after the hiatus that marks the M2 event.
This mPWP interval is correlated with the Poederlee Formation and Oorderen
Sands Member (of the Lillo Formation) located in Belgium (De Schepper et
al., 2009a; Louwye and De Schepper, 2010). The average TEX86 (10 ∘C) and U37K′ (16 ∘C) reconstructed
SSTs over the mPWP interval show good agreement with the PRISM3 model
reconstructions for February (10.4 ∘C) and August (16.7 ∘C; Dowsett et al., 2009). A common feature of the U37K′ and LDI SST records is the gradual warming between ∼3150 and 3050 ka (Fig. 6d), seen most clearly in the LDI record. Before the SST
warming from 3150 to 3050 ka, the percent of C32 diol decreases slowly (Fig. 6e),
indicating a decrease in freshwater discharge and/or an increased distance
to the coast. The low T/M ratios and the presence of a clay layer from
292 to 271 m in Fig. 4c (corresponding to 3155–3053 ka; Fig. 6) at this time
further indicate increased marine influence, likely as a result of sea level
rise. Differences in the absolute degree of warming recorded by the
U37K′ and LDI SST proxies could be attributed to the
different seasons in which they are produced, as well as by lateral transport
of certain biomarkers (Benthien and Müller, 2000; Ohkouchi et al.,
2002). For example, the change in currents in the North Sea after the M2
event, bringing in warmer waters from the North Atlantic, may have brought
alkenones and/or diols with a warmer signature to the SNSB. This would then
further contribute to the high SSTs reflected by the U37K′
and LDI proxies compared to those recorded by the TEX87H (Fig. 6).
Notably, where the TEX86H-derived SST record suggests relatively
stable winter temperatures, the LDI and U37K′ SST records
reflect highly variable SSTs during the mPWP (Fig. 6d). Such high
variability is also seen in all other currently available U37K′ SST records from the North Atlantic (8 ∘C,
Lawrence et al., 2009; 6 ∘C, Naafs et al., 2010; 14 ∘C, Clotten et al., 2018; Fig. S3) and has been explained
by a change in the strength of the NAC (Lawrence et al., 2009; Naafs et al.,
2010) and orbital forcing (Lawrence et al., 2009). In addition, high
variability in the U37K′ record from the Iceland Sea was
linked to the frequent occurrence of spring sea ice cover and ice-free
summers linked to freshwater input (Clotten et al., 2018). At Hank, the
influence of changes in the direction and strength of the NAC on
U37K′ SSTs cannot be unambiguously identified, despite the
reoccurrence of Operculodinium centrocarpum during the mPWP. Most of the variation may instead be
explained by the varying influence of freshwater input from the
proto-Rhine–Meuse River system in combination with the relatively shallow
coastal location of Hank, which makes it sensitive to fluctuations in
temperature. The influence of orbital forcing on pacing the variation in the
NAC (Naafs et al., 2010) and possibly the environmental conditions in the
SNSB require further analysis, which is currently not possible in the Hank
borehole due to the short length of the tuned interval.
In contrast to the variable marine climate, the terrestrial climate proxies
indicate that climate of land was fairly stable. The presence of
Taxodioidae-type pollen (Taxodium, Glyptostrobus) throughout most of the mPWP (Fig. 6c) indicates
that land temperatures were generally not low enough for prolonged winter
frosts. Minimum Taxodioidae-type pollen abundance of 10 % has been
associated with a mean temperature of the coldest month of > 5 ∘C (Fauquette et al., 1998). Both the MAT record (Dearing
Crampton-Flood et al., 2018) and the Taxodioidae-type pollen covary throughout the
whole record (Fig. 4d), and the absolute MATs and the increased proportion
of Taxodioidae-type pollen in the mPWP interval (Fig. 6) support the
presumed relatively stable climate conditions on land (Draut et al., 2003;
Lisiecki and Raymo, 2005).
Importantly, the new chronology for the Hank sediments provides an
opportunity to correlate the stratigraphy concept of the local (Netherlands)
qualitative Pliocene–Pleistocene Taxodioidae-type temperature curves
proposed by Zagwijn (1960, 1992). It should be noted that the original
terrestrial Pliocene stages as summarized by Zagwijn (1992) have not yet
been dated independently, and in the type area of the south-east
Netherlands, they likely represent much smaller intervals of time compared
to the Hank sequence. Zagwijn et al. (1992) inferred mean July temperatures
between 15 and 20 ∘C for the Reuverian, which were placed
approximately between 3.1 and 2.5 Ma, with short-lived cool pulses down to
∼12∘C that can also be recognized in the brGDGT
MAT record (Fig. 6f). Maximum Taxodioidae abundance and mean July
temperatures in excess of 20 ∘C were reconstructed for the
Brunssumian, placed approximately between 3.4 and 3.1 Ma. These reconstructed
summer temperatures compare broadly to the SSTs reconstructed using the
(presumably) partially summer-biased U37K′ proxy, which
range between ∼10 and 25 ∘C in the tuned interval
(Fig. 6d). Further correlation and dating studies in proximal boreholes and
marine sediment sequences will aid in deciphering the Reuverian A–C
substages and assigning absolute ages to the zonation of Zagwijn et al. (1992).
Conclusions
The age framework for the mid-to-late Pliocene of the southern North Sea
basin (SNSB) constructed here reveals that the M2 glacial event is represented as
a hiatus, confirming interpretations at proximal sites in Belgium and the
English North Sea coast. Sea surface temperatures were variable, which may
be caused by the sensitivity of the shallow Pliocene North Sea to climate
change and the influence of freshwater input on lipid biomarker SST proxies.
Nevertheless, the variability in SSTs matches that in all other currently
available SST records from the North Atlantic, indicating that the marine
realm was highly dynamic during the mPWP, probably as a result of shifting
currents caused by a reorganization/diversion of the North Atlantic Current.
Our terrestrial multi-proxy climate records show a highly consistent signal
between lipid biomarker temperatures and pollen assemblages, which show
stable terrestrial temperatures of 10–12 ∘C and the continued
presence of warm-adapted tree species during the end of the mPWP.
Importantly, the chronology presented here allows placing earlier
terrestrial temperature reconstructions for Pliocene NW Europe (Zagwijn et
al., 1992) in time. This indicates that the Reuverian Stage concept,
characterized by abundant Taxodioidae and Sciadopitys and rare Sequoia, is dated to ∼3.2–2.8 Ma. Further high-resolution analysis will attempt to resolve and
date the Reuverian A–C substages in this marine setting.
Data availability
The research data presented in this paper are available to download in the
Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/cp-16-523-2020-supplement.
Author contributions
EDCF, FP, and JSSD designed the research. CB and DS carried out the
geochemical analyses under supervision of EDCF, LN, FP, LL, and JSSD. DKM and
THD analysed and interpreted the palynological data. JtV provided seismic
interpretations. EDCF integrated the data and prepared the paper with
contributions from all authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors would like to thank Nico Janssen for processing palynological
samples, Giovanni Dammers and Natasja Welters for preparation of
foraminifera samples, Arnold van Dijk for help with isotope measurements,
and Anita van Leeuwen and Dominika Kasjaniuk for assistance in the organic
geochemistry lab. Stefan Schouten and Anchelique Mets at the Royal NIOZ
assisted with the analysis of long-chain diols. Two
anonymous reviewers and Stijn de Schepper are thanked for providing
constructive comments and suggestions that greatly improved the paper.
Financial support
This research has been supported by
funding from the Netherlands Earth System Science Centre (NESSC) from the Dutch Ministry for Education,
Culture and Science (gravitation grant no. NWO 024.002.001) to Jaap S. Sinninghe Damsté and Lucas Lourens.
Review statement
This paper was edited by Arne Winguth and reviewed by two anonymous referees.
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