The caldera-forming eruption of the Aniakchak volcano in the Aleutian Range
on the Alaskan Peninsula at 3.6 cal kyr BP was one of the largest Holocene
eruptions worldwide. The resulting ash is found as a visible sediment layer
in several Alaskan sites and as a cryptotephra on Newfoundland and Greenland.
This large geographic distribution, combined with the fact that the eruption
is relatively well constrained in time using radiocarbon dating of lake
sediments and annual layer counts in ice cores, makes it an excellent
stratigraphic marker for dating and correlating mid–late Holocene sediment
and paleoclimate records. This study presents the outcome of a targeted
search for the Aniakchak tephra in a marine sediment core from the Arctic
Ocean, namely Core SWERUS-L2-2-PC1 (2PC), raised from 57 m water depth in
Herald Canyon, western Chukchi Sea. High concentrations of tephra shards,
with a geochemical signature matching that of Aniakchak ash, were observed
across a more than 1.5 m long sediment sequence. Since the primary input of
volcanic ash is through atmospheric transport, and assuming that bioturbation
can account for mixing up to ca. 10 cm of the marine sediment deposited at
the coring site, the broad signal is interpreted as sustained reworking at
the sediment source input. The isochron is therefore placed at the base of
the sudden increase in tephra concentrations rather than at the maximum
concentration. This interpretation of major reworking is strengthened by
analysis of grain size distribution which points to ice rafting as an
important secondary transport mechanism of volcanic ash. Combined with
radiocarbon dates on mollusks in the same sediment core, the volcanic marker
is used to calculate a marine radiocarbon reservoir age offset
The Arctic is currently in a state of rapid transition as a result of its
sensitivity and amplified response to ongoing global climate warming
(Johannessen et al., 2004; Screen and Simmonds, 2010; Serreze and Barry,
2011). The last few decades have seen increased freshwater input to the
Arctic Ocean from rivers (Peterson et al., 2006), mass loss of glaciers and
ice caps (Gardner et al., 2011), and, most notably, a dramatic loss of
sea-ice cover (Cavalieri and Parkinson, 2012; Stroeve et al., 2014). The
observed sea-ice loss is most pronounced in the western Arctic Ocean, i.e.,
in the Chukchi and Beaufort seas (Comiso, 2002). To put the observed changes
in perspective, there is a strong need to investigate longer records of
climatic and oceanographic variability from natural archives such as sediment
cores. It is essential for this purpose to establish an accurate chronological
framework that will allow correlation to other marine and terrestrial records
and determine rates of change as well as leads and lags in the climate
system. Besides the use of lead and cesium isotope dating for the most recent
sediments, the standard approach for determining age and accumulation rates
in Holocene sediments is the analysis of radiocarbon (
Map of study area and all sites mentioned in this study. Colors of
markers – red: sites with Aniakchak CFE II tephra; blue: sites with modern
Another method used for dating sediment cores from the Arctic Ocean is the measurement of paleomagnetic secular variation, which relies on the movement of the Earth's north magnetic pole through time and its signature as remanent magnetization in sediments (Nowaczyk et al., 2001; St-Onge and Stoner, 2011). Although this is a promising technique applied successfully to high-temporal resolution sediment cores from the western Arctic Ocean (Barletta et al., 2008; Lisé-Pronovost et al., 2009), low sediment accumulation rates across much of the central Arctic (Backman et al., 2004; Jakobsson et al., 2014) preclude the use of paleosecular variation as a dating tool. Furthermore, there is a long-standing problem associated with the interpretation of paleomagnetic records in the Arctic Ocean, in which rapid and frequent reversals in polarity do not seem to be correlated with known geomagnetic excursions or Chron boundaries (Backman et al., 2004; Channell and Xuan, 2009; Jakobsson et al., 2000; O'Regan et al., 2008).
Tephrochronology is a widely applied method that relies on the presence of volcanic ash that can be used for absolute dating (Lowe, 2011). Depending on the magnitude of a volcanic eruption and the chemical composition of its tephra deposits, ash layers can be used to correlate marine, terrestrial and ice-core records across thousands of kilometers. Reports of tephra in Arctic Ocean sediments are very scarce and consist of a study from the Fram Strait (Zamelczyk et al., 2012) and preliminary findings of tephra in the Chukchi Sea (Ponomareva et al., 2014). The aim of this study is to obtain an estimate of the local radiocarbon reservoir offset in the Chukchi Sea during the late Holocene. A marine sediment core from Herald Canyon with high sediment accumulation rates is used for this purpose by combining accelerator mass spectrometry (AMS) radiocarbon dates on mollusks from 14 different depths with the Aniakchak tephra of approximately 3600 cal yr BP as an absolute age marker.
The Aniakchak volcano of the Aleutian Range on the Alaskan Peninsula
(56.88
Since the ash occurs at many sites, estimates for its age have been obtained
using different methods. The precise age of the Aniakchak CFE II eruption,
however, is currently the subject of debate (Bacon et al., 2014; Blackford et
al., 2014; Davies et al., 2016; Kaufman et al., 2012; Pearce et al., 2004). A
recent review (Davies et al., 2016) concluded that the age of the event,
based on all available radiocarbon dates, does not agree with the age
obtained from the Greenland ice cores, where the age appears to be too old by
several decades. Here we argue that the radiocarbon dates from the Alaskan
lakes are in fact compatible with the ice-core ages, when taking into account
recent work that quantifies a possible offset between the IntCal13
radiocarbon timescale and the GICC05 ice-core timescale (Adolphi and
Muscheler, 2016; Muscheler et al., 2014). Based on Adolphi and
Muscheler (2016), the difference between the Greenland ice-core chronology
and the radiocarbon calibration curve, expressed as IntCal13
The Chukchi Sea is a marginal sea of the Arctic Ocean and covers the large
shallow shelf bordered by Alaska to the east and the Chukotka Peninsula to
the west (Fig. 1). It is separated from the Pacific Ocean by the Bering
Strait, stretches over ca. 620 000 km
Radiocarbon date list for core SWERUS-L2-2-PC1. All dates are calibrated using the Marine13 calibration curve (Reimer et al., 2013). The top two dates are reported in percent modern carbon (pMC) rather than years. BP: before present (AD 1950).
All results presented in this study are based on lithological, geochemical and microscopic analysis of samples from a single sediment core in the western Chukchi Sea.
Core SWERUS-L2-2-PC1 (hereafter 2PC) is an 824 cm long piston core taken
from 57 m water depth at 72.52
Accelerator mass spectrometry (AMS) radiocarbon measurements were made on
mollusks from 14 different depths in Core 2PC (Table 1). From three samples,
the mollusks were broken in half and sent to different radiocarbon
laboratories to ensure reproducibility. This resulted in a total of 17 AMS
measurements and 14 effectively dated horizons in the core (Table 1).
Radiocarbon dates were converted to calendar ages using the Marine13
calibration (Reimer et al., 2013) and the Oxcal 4.2 program (Bronk Ramsey,
2008, 2009). Initially a value of
Tephra concentrations in Core 2PC were determined between 546 cm and the
bottom of the core at 824 cm, specifically to target the Aniakchak CFE II
eruption. This depth interval was chosen to ensure that sediments with an
approximate age of 3600 cal yr BP were included in the analysis, as well
as older samples to quantify any possible background levels of tephra shards.
A series of consecutive cuboid 50 mm
Major oxides compositions of tephra shards in core 2PC from the samples listed in Table 2, compared to data from the literature. All values are percentages, normalized to 100 %.
The major elemental composition of tephra shards was determined using an
electron probe microanalyzer (EPMA) on selected samples. Material from four
different depths (Fig. 2, Table 2) was mounted in epoxy on thin section
slides, polished and coated with carbon prior to analysis. The measurements
were made on a JXA-8530F JEOL Superprobe at Uppsala University using a
15 kV, 4 nA beam of 10
Major element geochemistry of tephra shards in core 2PC measured by EPMA. For each depth, the concentrations are averages of 10 individual shard measurements. All values are in percentages and normalized to sum to 100 %. The complete dataset with all individual measurements is in Table S3.
Analysis of grain size distribution was performed on a specific interval in
the lower part of Core 2PC, based on the obtained profile of tephra
concentrations. The aim was to investigate a possible relationship between
grain size distribution and tephra abundances by selecting an interval with
both high and low concentrations of volcanic shards. Between 646 and 747 cm
depth, grain size distributions from 2
The combination of an absolute age marker from a volcanic eruption and
radiocarbon age estimate is used to calculate the radiocarbon marine
reservoir age. If the radiocarbon age estimate for the volcanic horizon is
too old compared to the absolute age of eruption, a positive
The 14 AMS radiocarbon measurements do not contain any reversals or obvious
outliers, and indicate a continuous sequence of nearly constant sediment
accumulation rate (Table 1). The ages of duplicate measurements from
different facilities on split mollusks (at 233.0, 646.5, and 760.5 cm)
returned fully consistent results within 1 standard deviation of error. The
duplicate measurements from those samples were combined to slightly reduce
the associated uncertainties of the obtained radiocarbon ages (Table 1). The
two topmost samples contain radiocarbon content higher than “modern”,
indicating deposition after the onset of thermonuclear bomb testing during
the 1950s. The radiocarbon analyses indicate that Core 2PC represents just
over 4000 years of deposition with an average sediment accumulation rate of
about 194 cm kyr
Grain size distribution
Tephra was not observed through visual inspection or from smear slides of
bulk sediment. Observations and quantifications of tephra, as described
below, were based on microscopic studies of samples after sieving and density
separation. Tephra shards observed in Core 2PC range in size from the minimum
sieve size of 25 up to 340
The grain size distribution is relatively constant throughout the
investigated interval (646–747 cm) (Fig. 4). The distribution is dominated
by the silt size fraction (2–63
The geochemistry of the tephra shards in Core 2PC shows an excellent
agreement with that of other, more proximal, deposits of the Aniakchak CFE II
eruption. A comparison between all major oxide concentrations and those of
the rhyolitic, high-Si fraction of Aniakchak layers from several sites in
Alaska (Fig. 1) is presented in Fig. 3. The SiO
When studying tephra deposits in sediments, one of the major challenges is to identify the isochron depth, i.e. the sediment layer corresponding to the timing of the volcanic eruption. This is especially true in marine environments where secondary processes can play a major role in redistributing sediments and obscuring the original signals (Griggs et al., 2015; Lowe, 2011). In an idealized setting, tephra particles are deposited after a volcanic eruption and rapidly buried and preserved, resulting in a clean marker horizon where the bottom contact is isochronous to the onset of the eruption. In reality, the tephra layer is often affected by bioturbation, which redistributes the tephra in the sediment column, as well as secondary transport mechanisms which can produce a delayed signal (Davies et al., 2010; Lowe, 2011). Despite these processes which broaden the tephra distribution, the primary direct deposition of tephra may remain to be the dominating input mechanism and would result in a peak concentration at the isochron depth (Lowe, 2011). If, however, the reworked signal dominates over the primary deposition, the peak tephra concentration may significantly lag the actual time of eruption.
Calculation of marine reservoir age offset
Tephra was found in all studied samples of Core 2PC, which indicates that secondary processes should definitely be taken into consideration when interpreting the signal. Although the presence of tephra is continuous, there is significant variability in the actual concentrations and a clear background signal preceding the increase associated with the Aniakchak CFE II eruption. The highest peak of the tephra distribution in Core 2PC occurs at 653.5 cm depth (Fig. 2), which is 58 cm above the first major increase in tephra concentration above background levels at 711.5 cm. The depth of the mean mixed layer has not been determined in Core 2PC, but other studies in the region observed mixing depths of 5–10 cm on the Chukchi Shelf (Baskaran and Naidu, 1995; Clough et al., 1997; McKay et al., 2008). These values are well above the average maximum mixing depth of approximately 3 cm in the Arctic Ocean (Clough et al., 1997), and thus indicate that bioturbation may play a significant role in this environment. In Core 2PC, however, no visible traces of bioturbation were observed during visual inspection. Furthermore, a theoretical mixing depth of maximum 10 cm would not be able to explain an offset of 58 cm between the depths of first occurrence and peak tephra concentration. Since a downward redistribution of this magnitude caused by bioturbation is thus not possible in this environment, we argue that the deepest occurrence is the most likely depth for the eruption isochron.
Our placement of the isochron is thus based on the principle of first shard occurrence, although some mixing by bioturbation can not be ruled out entirely. To determine the uncertainty associated with our calculated reservoir age, we include a maximum mixing range of 15 cm. When assuming a maximum bioturbation depth estimate of 15 cm, this results in a depth range of the isochron from 711.5 to 696.5 cm (Fig. 2).
This interpretation thus also implies that the majority of tephra shards must have been delivered to the core site after the eruption by secondary processes, i.e. reworking of primary tephra deposits. The main transport mechanisms for this region are suspended and bed load transport by currents and sea-ice transport, either by suspension freezing or anchor ice formation (Darby et al., 2009). Grain size analysis can be useful in marine tephrochronology since anomalous patterns may be indicative of reworked sediments (Lowe, 2011). The grain size distribution in samples from Core 2PC shows that peaks of tephra concentration correspond to intervals with increased input of coarse material (Fig. 4). Despite the small concentrations of the coarser grain fractions, this correlation is a clear indicator of ice rafting during these intervals. This mechanism to transport tephra from distal primary deposits to the core site may explain the elevated tephra concentrations during these intervals.
Although similar lags between volcanic eruptions and tephra burial in
sediments caused by secondary processes such as ice transport have been found
in other studies from different regions (Austin et al., 1995; Brendryen et
al., 2010; Lowe, 2011), our findings are the first report of geochemically
identical tephra shards in more than 1 m of marine sediment. On the
Hebridean Shelf off NW Scotland, Austin et al. (1995) placed the isochron
depth for the Vedde ash layer at its first occurrence and argued for
continuous reworking of larger-grained materials, explaining the peak ash
concentration 20 cm higher in the core. With high sedimentation rates on the
order of 1 cm yr
The offset between the age based on the radiocarbon age model and the
absolute age of the volcanic eruption can be used to determine the reservoir
age correction at the isochron depth (Ascough et al., 2005; Eiríksson et
al., 2004; Olsen et al., 2014). Here, the volcanic age marker of
3572
Final age–depth relationship
This value represents the reservoir age at the time of the eruption and is
not necessarily constant throughout the entire late Holocene. The calculated
value, however, matches very well with the limited modern data available from
mollusks collected before atmospheric radiocarbon contamination by testing of
nuclear bombs in the 1950s (Druffel and Linick, 1978). A total of 11 such
samples exist from the region, all from coastal Alaska: 4 offshore of Point
Barrow, 3 south of Wainwright, and 4 just outside Teller on the other side of
the Bering Strait (Fig. 1), with an average value of 468
Since the depth of the tephra isochron is not precisely determined, but
rather represented as a depth range, it is not possible to include the
absolute age marker with its associated low uncertainty directly into the age
model. The final age model is therefore constructed based solely on
radiocarbon dates and includes the Aniakchak CFE II indirectly by using a
Core SWERUS-L2-2-PC1 (2PC) from Herald Canyon in the Chukchi Sea contains a
continuous sequence deposited at high sediment accumulation rates over the
last 4250 years and has the potential to be used for studying late Holocene
climate and ocean variability at decadal resolution in a region where no such
data exist. The results presented here focus on a single volcanic eruption,
the 3.6 ka Aniakchak CFE II, in a single sediment core in the Chukchi Sea
and therefore much work can be done to expand on this study. The upper half
of the core contains potentially more volcanic age markers, although the
tephra signal becomes more complex to interpret due to reworking of older
eruptions. The earlier caldera-forming eruption of the Aniakchak volcano
during the early Holocene, Aniakchak CFE I, may also be found in other
records which extend further back in time. Based on our findings in Core 2PC
and the geographic spread of the Aniakchak CFE II on land, this ash should
also be present in other marine records of the Chukchi and Beaufort seas. If
identified in other marine sediments from the region, the ash layer can be
used as an age-equivalent marker between those sites and will greatly reduce
the errors of their associated chronologies. Based on the presence of the
Aniakchak CFE II tephra, the radiocarbon reservoir age
All data presented in this manuscript can be found in the Supplement and will also be made available online through the Bolin Centre Database at
The authors declare that they have no conflict of interest.
Our gratitude goes to the entire crew and scientific party onboard R.V.