Stable isotope and calcareous nannofossil assemblage record of the late Paleocene and early Eocene ( Cicogna section )

We present records of stable carbon and oxygen isotopes, CaCO3 content, and changes in calcareous nannofossil assemblages across an 81 m thick section of upper Paleocene–lower Eocene marine sedimentary rocks now exposed along the Cicogna Stream in northeast Italy. The studied stratigraphic section represents sediment accumulation in a bathyal hemipelagic setting from approximately 57.5 to 52.2 Ma, a multi-million-year time interval characterized by perturbations in the global carbon cycle and changes in calcareous nannofossil assemblages. The bulk carbonate δC profile for the Cicogna section, once placed on a common timescale, resembles that at several other locations across the world, and includes both a long-term drop in δC and multiple short-term carbon isotope excursions (CIEs). This precise correlation of widely separated δC records in marine sequences results from temporal changes in the carbon composition of the exogenic carbon cycle. However, diagenesis has likely modified the δC record at Cicogna, an interpretation supported by variations in bulk carbonate δO, which do not conform to expectations for a primary signal. The record of CaCO3 content reflects a combination of carbonate dilution and dissolution, as also inferred at other sites. Our detailed documentation and statistical analysis of calcareous nannofossil assemblages show major differences before, during and after the Paleocene–Eocene Thermal Maximum. Other CIEs in our lower Paleogene section do not exhibit such a distinctive change; instead, these events are sometimes characterized by variations restricted to a limited number of taxa and transient shifts in the relative abundance of primary assemblage components. Both long-lasting and short-lived modifications to calcareous nannofossil assemblages preferentially affected nannoliths or holococcoliths such as Discoaster, Fasciculithus, Rhomboaster/Tribrachiatus, Sphenolithus and Zygrhablithus, which underwent distinct variations in abundance as well as permanent evolutionary changes in terms of appearances and disappearances. By contrast, placoliths such as Coccolithus and Toweius, which represent the main component of the assemblages, were characterized by a gradual decline in abundance over time. Comparisons of detailed nannofossil assemblage records at the Cicogna section and at ODP Site 1262 support the idea that variations in the relative and absolute abundances, even some minor changes, were globally synchronous. An obvious link is through climate forcing and carbon cycling, although the linkages between variations in calcareous nannoplankton, changes in δC records and oceanography will need additional work. Published by Copernicus Publications on behalf of the European Geosciences Union. 884 C. Agnini et al.: Stable isotope and calcareous nannofossil assemblage record


Introduction
A remarkable interval of global warming occurred from the middle Paleocene to the early Eocene, between approximately 59 and 51 million years ago (Ma).This inference comes from a variety of proxies (Huber and Caballero, 2011;Hollis et al., 2012), including the stable oxygen isotope (δ 18 O) composition of benthic foraminifera (Fig. 1).The precise timing of the long-term temperature rise remains somewhat unconstrained, because absolute ages across the early Eocene remain unsolidified.Throughout this work, we assume that the option 1 (WO-1) timescale presented by Westerhold et al. (2008) is correct (Table 1), but acknowledge that an offset of ca.400 kyr may occur within the time interval of interest (Vandenberghe et al., 2012).Debate also surrounds the magnitude and distribution of the temperature warming.Earth's surface temperatures, at least at high latitudes and in the deep sea, seem to have risen by at least 6 • C from ca. 59 to 51 Ma (Zachos et al., 2008;Bijl et al., 2009;Huber and Caballero, 2011;Hollis et al., 2012).Indeed, the latter date marks the acme of the Early Eocene Climatic Optimum (EECO), the warmest sustained time interval of the Cenozoic (Zachos et al., 2008;Cramer et al., 2009;Hollis et al., 2012).Such a rise in temperature is not obvious at low latitudes with current data (Pearson et al., 2007;Huber and Caballero, 2011).
The early Paleogene in general, and the hyperthermals in particular, has attracted considerable geoscience research.On one level, this is because these time intervals represent a range of possible past analogues for understanding the effects of global warming and massive carbon emissions (see Keeling and Whorf, 2004;Zachos et al., 2008).On another level, this is because the long-term and short-term temperature and carbon cycle perturbations provide new perspectives for how systems on Earth's surface operate.The PCIM probably represents a tremendous storage of 13 C-depleted carbon somewhere on Earth's shallow surface (Shackleton, 1986;Kurtz et al., 2003;Komar et al., 2013).In turn, the CIEs probably signify rapid and large inputs of 13 C-depleted carbon into the ocean and atmosphere (Dickens et al., 1997;Lourens et al., 2005;Zeebe et al., 2009).The middle Paleocene through early Eocene shows us that Earth's climate and carbon reservoirs were extremely dynamic during past times of global warmth.However, the composition and whereabouts of large quantities of transferable 13 C-depleted carbon (e.g., seafloor methane, peat, permafrost) remain uncertain (above references).Indeed, it is not clear if and how the long-term and short-term carbon cycle perturbations were related to one another, or to Earth surface temperatures.
The above context presents a series of basic questions to the geoscience community.Two of these are the focus of our study.(1) What is the correct template for understanding carbon cycling during the early Paleogene?Major changes in fluxes of 13 C-depleted carbon to the ocean or atmosphere should give predictable and coherent signals in the δ 13 C of carbon-bearing phases across Earth, as well as the distribution of carbonate dissolution on the seafloor.This is not yet established.For example, several recently published δ 13 C records (Kirtland-Turner et al., 2014;Slotnick et al., 2015a;Payros et al., 2015) do not precisely correlate with those at other locations (Cramer et al., 2003;Zachos et al., 2010;Slotnick et al., 2012Slotnick et al., , 2015b)), at least with available stratigraphy.(2) How did marine calcifying organisms respond to major early Paleogene perturbations in temperature and carbon cycling, both in terms of evolution and preservation?The prominent changes in temperature and carbon fluxes almost assuredly caused large variations in seawater pH and carbonate ion concentration (CO 2− 3 ; Dickens et al., 1997;Zachos et al., 2005;Kump et al., 2009;Zeebe et al., 2009;Leon-Rodriguez and Dickens, 2010;Hönisch et al., 2012;Pälike et al., 2012), although the response should depend on location and carbon fluxes involved (Dickens, 2000;Zeebe and Westbroek, 2003;Komar et al., 2013).Such changes might also affect the ability of living organisms to calcify (Riebesell et al., 2000(Riebesell et al., , 2008;;Kleypas et al., 2006;Iglesias-Rodriguez et al., 2008;Stillman and Paganini, 2015), which might impact the fossil record (Agnini et al., 2006;Raffi and De Bernardi, 2008;Erba et al., 2010;Hönisch et al., 2012).
While numerous studies have examined calcareous nannofossils across the PETM from different perspectives (e.g., Bralower, 2002;Stoll and Bains, 2003;Gibbs et al., 2006a, b;Agnini et al., 2007a;Mutterlose et al., 2007;Bown and Pearson, 2009;Jiang and Wise, 2009;Self-Trail et al., 2012), the relationship between these organisms and carbon cycle perturbations before and after this short-lived warming episode remain poorly documented (Gibbs et al., 2012).It seems possible that the high rate of calcareous nannofossil taxonomic evolution (appearances and extinctions), as well as distinct changes in calcareous nannofossil abundance patterns, may provide excellent stratigraphic control across the early Paleogene (Bukry, 1973;Perch-Nielsen, 1985;Backman, 1986;Agnini et al., 2014).Moreover, if the exact relationship between changes in nannofossil assemblages and global carbon cycle perturbations were known, key time intervals could be rapidly identified for more detailed work.Finally, changes in calcareous nannofossils across the early Paleogene provide insights about the response of an important part of the sediment forming marine biota to changes in climate and carbon cycling.
content) and calcareous nannofossil census data from the Cicogna section in northeast Italy (Figs. 2, 3).These data are compared with similar information from sites 1262 and 577.We show that the Cicogna section provides an important template for understanding potential relationships between climate, carbon cycling and the biotic evolution of calcareous nannoplankton.

The Cicogna section
The Cicogna section crops out along the Cicogna Stream near the village of Tassei in Belluno Province, northeast Italy (Fig. 3).From a regional geological perspective, the sedimentary rocks of this section belong to the Belluno Basin.This basin represents part of a paleogeographic domain that formed when Jurassic rifting created a series of N-S-oriented structural highs (platforms) and lows (basins), which persisted through much of the Paleogene (Bernoulli and Jenkyns, 1974;Bernoulli et al., 1979;Winterer and Bosellini, 1981).Importantly, from the Cretaceous to the middle-late Eocene, expanded deep sea sediment successions accumulated within the basins at nominally 30 • N latitude (Stefani and Grandesso, 1991;Agnini et al., 2006Agnini et al., , 2011;;Zattin et al., 2006).The Cicogna section consists of two lithostratigraphic units (Fig. 3).The lower portion is a well-exposed upper Paleocene and lower Eocene unit referred to as Scaglia Rossa sensu lato (Figs. 3, 4;Giusberti et al., 2007;Dallanave et al., 2009).Based on benthic foraminiferal assemblages, the early Paleogene marls of this unit represent lithified pelagic and hemipelagic sediment that accumulated at middle to lower bathyal water depth, likely between 600 and 1000 m and not deeper than 1500 m (Giusberti et al., 2007(Giusberti et al., , 2016)).The upper portion is a thick early to middle Eocene unit called the Belluno Flysch (Figs. 3,4).This unit represents a synorogenic deposit on the flanks of the former Trento and Friuli platforms (Grandesso, 1976;Doglioni and Bosellini, 1987).
Once corrected for bed strike and dip (ca.315 • N; ca.45 • ) and bends in the stream, the Scaglia Rossa at Cicogna measures 80 m in terms of stratigraphic height (Dallanave et al.,   Dallanave et al. (2009); CN biozones are also reported.Orange and yellow bands mark major δ 13 C excursions.Dashed lines indicate minor CIEs that have been labeled elsewhere (e.g., E1/E2, F and G; Cramer et al., 2003), whereas dotted lines indicate minor changes in δ 13 C that appear to occur also at ODP Site 1262 (see also Fig. 5).

2009
).The Belluno Flysch measures 1 m in the Cicogna section.Furthermore, the section of interest can be subdivided into several subunits (Fig. 3).The lower 20 m is comprised of distinctive alternating beds of gray-greenish to purple marls and calcareous marls, the latter defined by carbonate contents higher than 60 % (Fig. 3c).This is overlain by approximately 9 m of pink-red marls with much less lithologic alternation.At 28.7 m, the sedimentary package is broken sharply by an approximately 3 m thick red to brownish-red interval of clayey marls with sporadic gray-green centimeter-scale spots and lenses (Fig. 3f, g).This has been called the Clay Marl Unit (CMU), and records the core of the prominent negative δ 13 C excursion associated with the PETM at multiple outcrop sites within the Belluno Basin (Agnini et al., 2006(Agnini et al., , 2007a;;Giusberti et al., 2007).Above the CMU, from 31.7 to 39.2 m, the section continues with deposition of rhythmic alternations of marls and calcareous marls (Fig. 3g).Above this 8.5 m thick interval, at ca. 40.5 m, spatic calcite crystals occur.Generally, couplets of marl and calcareous marl couplets become less evident from 40.5 until 54 m, where such couplets reoccur (Fig. 3d).At 75.5 m, a thin calcarenitic bed is encountered, presaging the onset of the Belluno Flysch.This turbidite is followed by a temporary return to hemipelagic sedimentation that ends at 80.6 m.Above, sedimentation of the Belluno Flysch begins in earnest (Figs.3b, 4).
The basic stratigraphy of the Scaglia Rossa in the Cicogna section, including both polarity chron boundaries and key calcareous nannofossil biohorizons, has been published (Giusberti et al., 2007;Dallanave et al., 2009) (Okada and Bukry, 1980) or CNP10 to CNE4 (Agnini et al., 2014).Thus, the section represents a 5.3-million-year-(Myr) long time interval, from about 57.5 to 52.2 Ma on the W0-1 timescale.This also implies an average sedimentation rate (SR) of ca. 15 m Myr −1 .Although the deposition of hemipelagic sediment might suggest relatively constant SRs over time, the PETM and possibly other hyperthermal events in the Belluno Basin were characterized by higher SRs (Giusberti et al., 2007;Agnini et al., 2009;Tipple et al., 2011;Krishnan et al., 2015).
The Scaglia Rossa at Cicogna appears to record fairly continuous sediment accumulation at moderately high deposition rates.This is important because it affords longer time duration than most shallow ocean sites, greater time resolution than most deep ocean sites (Fig. 2), and an overall different environmental setting.Many early Paleogene records, especially those from paleo-shelf environments, such as in Egypt (e.g, Aubry and Salem, 2012) and New Jersey (Mixon and Powars, 1994;Harris et al., 2010), or from early deep sea drilling expeditions, such as in the Indian Ocean (Slotnick et al., 2015b), are discontinuous, because of either hiatuses or core gaps.Much of the detailed work and current understanding of stable carbon isotope stratigraphy and calcareous nannofossil variations across the broad early Paleogene, therefore, has come from deep-sea drilling sites with multiple holes but slow sedimentation rates, although we note the work in Clarence Valley, New Zealand (Fig. 2), another area that contains several paleo-slope sections with moderately high sedimentation rates (Nicolo et al., 2007;Slotnick et al., 2012Slotnick et al., , 2015b;;Dallanave et al., 2015).For the Cicogna section, detailed stable isotope and CaCO 3 records are currently lacking, as well as detailed calcareous nannofossil assemblage information, which we present here.

Samples
A total of 492 samples were chiseled from outcrops along the section.Samples were selected so as to be as fresh and unaltered as possible.This included chipping off weathered surfaces while in the field.Each sample was calibrated to height (Fig. 4).Samples then were split, with one portion powdered in an agate ball mill, and subsequently freeze-dried.

Geochemistry
Each of the powdered samples was analyzed for bulk sediment stable isotope composition at the Stable Isotope Laboratory, University of Southampton, UK.A known mass (∼ 80 µg) was placed into a headspace vial, dried overnight, and flushed with helium.To each sample, 10 mL of 100 % phosphoric acid was added and allowed to react.The lib-erated CO 2 gas was measured using an EUROPA Scientific GEO 20-20 mass spectrometer fitted with a microCAPS for carbonate analysis.Results are reported in standard delta notation relative to Vienna Pee Dee Belemnite (VPDB).An in-house standard of Carrara marble, calibrated to NBS-19 limestone, was measured multiple times to evaluate accuracy and precision.The external analytical precision (1σ ), based on these replicate analyses, was 0.028 ‰ for δ 13 C and 0.057 ‰ for δ 18 O.
The amount of CaCO 3 in each sample was calculated from the beam height response during isotope mass spectrometer measurements (Spofforth et al., 2010).The liberated CO 2 gas, when squeezed up in the bellows, is measured and generates a current, the beam height.The pressure of CO 2 gas is directly proportional to the beam height and therefore the mass of carbonate in the sample.Over 100 samples of pure CaCO 3 , with masses between 200 and 480 µg, were analyzed to establish a linear relationship between beam height and carbonate content (CaCO 3 = mx + b; R 2 = 0.94-0.99).Results were validated by analyzing 30 samples on a C-H-N-O elemental analyzer.

Calcareous nannofossils
The un-powdered sample split was examined for calcareous nannofossils.Raw sediments were processed to prepare standard smear slides (Bown and Young, 1998).To assess the reproducibility of our counting methods in every single sample, a pivotal sample was prepared 10 times by two different operators.Repeated counts of the identical sample performed by different analysts gave similar results (sd < 2-5 %).Particle density estimates (Baccelle and Bosellini, 1965) were not carried out because samples have a high range in the terrigenous content (22 to 90 %).An increase or decrease in the silicoclastic component is mainly related to the major or minor efficiency of the chemical and mechanical weathering on land (Agnini et al., 2009).In the studied sediments, the variation in the amount of the terrigenous content through time has modified the density of the allochemic particle component.Consequently, calcareous nannofossil absolute abundances could not be estimated correctly using a homogeneous/constant particle density or by weighing the same amount of sediment for each smear slide.However, the scope of semi-quantitative counts performed in this study is to recognize the precise position of biostratigraphic biohorizons rather than use these data as a proxy of the paleoproductivity of taxa.Essentially, the identification of the appearance or disappearance of any given taxon is not affected by its stratigraphic abundance pattern, which obviously reduces the negative effect of the variable abundance of the silicoclastic component throughout the section.Samples were examined under a Zeiss light microscope at 1250× magnification.Calcareous nannofossils were determined using taxonomy proposed by Aubry (1984Aubry ( , 1988Aubry ( , 1989Aubry ( , 1990Aubry ( , 1999b)), Perch-Nielsen (1985) and Bown (2005).(Zachos et al., 2010), and DSDP Site 577 (Cramer et al., 2003;Dickens and Backman, 2013).All three sites have independently derived nannofossil biohorizons, polarity chrons and δ 13 C records, which account for subtle temporal offsets.Color bands and symbols are the same as in Fig. 4. Note the missing record at Site 577 that corresponds to known core gaps.
A total of 200 samples were examined, providing an average time resolution of ca. 25 kyr.A preliminary qualitative estimate of the abundance and preservation state of calcareous nannofossil assemblages was performed for all samples.An initial large batch (185) was analyzed primarily to provide biostratigraphic control for the Cicogna section, and the basic results have been presented by Dallanave et al. (2009).We re-checked and refined the positions of some biohorizons by examining 15 additional samples, primarily across some of the CIEs, such as B1/B2, PETM, H1 and H2, and K/X (Cramer et al., 2003).The calcareous nannofossil biostratigraphic schemes used by Dallanave et al. (2009) were those of Martini (1971) and Okada and Bukry (1980).The new zonal scheme of Agnini et al. (2014) is also used here.Biohorizon nomenclature follows that given by Agnini et al. (2014): base (B), base common (Bc), top (T) and top common (Tc).
Calcareous nannofossil biostratigraphic results are based on semi-quantitative analyses, which is based on counts of the number of specimens of selected taxa present in a prefixed area, 1 mm 2 or three long traverses (modified after Backman and Shackleton, 1983).Calcareous nannofossil paleoecological results are instead based on relative abundances of calcareous nannofossil taxa (percent of the total assemblage), calculated from counts of at least 300 specimens.
To capture changes in calcareous nannofossil assemblages we also use a statistical approach.Principal component analysis (PCA) was preferred to other methods, such as the non-metric multidimensional scaling (MDS) procedure, for which a small number of axes are chosen prior to the analysis and the data are fitted to these dimensions (Hammer et al., 2001).However, non-metric MDS results were performed and are available as Supplement (Fig. S2).Multivariate analysis of variance (MANOVA) was carried out on our data set to determine whether significative differences are present among the three groups of samples recognized with PCA.
PCA and MANOVA were performed on the percentages of 15 subgroups using the statistical software package PAST ver.2.17c (Hammer et al., 2001).The former analysis is often used for examining paleontological data (e.g., Buccianti et al., 2006;Kucera and Malmgren, 1998;Watkins and Self-Trail, 1992;Thibault and Gardin, 2010;Marino et al., 2008;Bordiga et al., 2015), as it can point out hypothetical variables (components) that explain much of the variance in a multidimensional data set.The first principal component accounts for the most variability in any data set examined.Each succeeding component has the highest variance possible relative to the preceding components (Hammer et al., 2001).This method also increases the symmetry, homoscedasticity and linearity of the data set (Aitchison, 1986).The chosen subgroups were Chiasmolithus, Coccolithus, Ellipsolithus, Discoaster, Ericsonia, Fasciculithus, Girgisia, Octolithus, Prinsius, Sphenolithus, Toweius, Rhomboaster/Tribrachiatus, Zyghrablithus, reworked forms, and "others".

Carbon isotopes
The bulk rock δ 13 C record for the Cicogna section can be described, in a general sense, as a long-term decrease of approximately 3 ‰, punctuated by a series of negative CIEs (Fig. 4).The most prominent low in δ 13 C coincides with the CMU.
Previously established polarity chron boundaries and key calcareous nannofossil biohorizons at the Cicogna section (Dallanave et al., 2009) provide a very good stratigraphic framework.Once placed onto a common timescale, in this case WO-1 (Westerhold et al., 2008), the δ 13 C record at Cicogna is fairly similar to those records generated using upper Paleocene and lower Eocene marine carbonate at other locations (Cramer et al., 2003;Zachos et al., 2010;Slotnick et al., 2012).This includes, for example, bulk carbonate δ 13 C records at ODP Site 1262, and DSDP Site 577 (Fig. 5) The relatively high δ 13 C values near the base of the Cicogna section document the late stages of the PCIM, which was centered within C25r (Fig. 1).The overall drop in δ 13 C across the section marks the long-term global decrease in δ 13 C that lasted through Chron C24n (Fig. 1).The record contains multiple negative shifts in δ 13 C.There is, however, an intriguing difference: across the Cicogna section, the long-term 3 ‰ shift in bulk carbonate δ 13 C values is generally offset from that in bulk carbonate δ 13 C records at sites 1262 and 577 by approximately −1 ‰.
The superimposed CIEs are considered to correspond to CIEs found in δ 13 C records from elsewhere, some of which represent known or inferred hyperthermal events (Cramer et al., 2003;Lourens et al., 2005;Nicolo et al., 2007;Zachos et al., 2010;Slotnick et al., 2012).There are three pairs of CIEs below the CMU (Fig. 4), as well as during the initial upper Paleocene long-term decline in δ 13 C.These correspond to the B1/B2, C1/C2 and D1/D2 CIEs documented by others (Cramer et al., 2003;Zachos et al., 2010).Notably, at Site 1262, the B1/B2 CIEs occur during middle C25n, and the C1/C2 CIEs occur at the start of C24r (Fig. 5).The same is true at Cicogna.Interestingly, at Cicogna, the B2 and C2 CIEs show greater magnitudes than the B1 and C1 CIEs, and these paired excursions are more pronounced than at all other locations examined to date.An additional paired CIE occurs in the uppermost Paleocene (Fig. 4).This may correlate to a fourth set of late Paleocene CIEs documented at Site 1262 (Zachos et al., 2010).
The lower Eocene portion of the δ 13 C record at Cicogna (Fig. 4) begins at the CMU, which marks the PETM (Giusberti et al., 2007;Dallanave et al., 2009).As at many locations, the PETM is characterized by a prominent negative CIE.The shift in δ 13 C at Cicogna is approximately −2.5 ‰, a decrease that begins abruptly at 28.7 m and returns more gradually to near pre-excursion values by about 33 m.From approximately 33 to 54 m, the δ 13 C curve shows a relatively smooth trend.At 54 m, a pair of CIEs begin, with the first pair having a magnitude of about 1.0 ‰.These are the H1/H2 events (Cramer et al., 2003), which occurred in the upper part of Chron C24r (Lourens et al., 2005;Zachos et al., 2010;Dickens and Backman, 2013;Dallanave et al., 2015).Above the H1/H2 CIEs, and within Chron C24n, are a series of smaller (0.4 to 0.6 ‰) CIEs.Those at approximately 60, 65 and 72 m are correlated with the I1/I2, J and K/X events, respectively.In summary, the δ 13 C record at Cicogna correlates with that at ODP Site 1262 (Zachos et al., 2010) and DSDP Site 577 (Dickens and Backman, 2013; Fig. 5), as well as at several other locations (Cramer et al., 2003;Slotnick et al., 2012Slotnick et al., , 2015b)).This is important because it enables comparison and discussion between widely separated sedimentary records within a firm temporal framework.

Oxygen isotopes
The δ 18 O values range from −1.08 to −3.64 ‰ with a mean value of −1.96 ‰ and a standard deviation (1σ ) of 0.50 ‰ (Fig. 4).However, at the broad scale, δ 18 O increases upsection, with Paleocene samples averaging −2.10 ‰ and Eocene samples averaging −1.89 ‰.This trend is noteworthy because δ 18 O values should decrease upsection if the composition of the CaCO 3 was principally reflecting rising global temperatures through the early Eocene.The 1σ of δ 18 O values also increases upsection, being 0.33 ‰ across Paleocene samples and 0.56 ‰ across Eocene samples.

Carbonate content
The CaCO 3 content varies between 9.4 and 77.7 % across the sample suite, with a mean value of 54.3 % and a 1σ of 8.2 % (Fig. 4).Two important findings emerge from the CaCO 3 content record.First, from 39 to 54 m, where we find limited variance in the δ 13 C curve, CaCO 3 content averages 52.1 % with a 1σ of 4.9 %.Thus, while the average is similar to that calculated for the entire section, the standard deviation is much less.At Site 1262, the corresponding time interval is also characterized by limited variance in δ 13 C values and carbonate contents, the latter inferred from the abundance of Fe counts in XRF scans (Zachos et al., 2010).Second, across all samples, the CaCO 3 content covaries somewhat (r = 0.29) with δ 13 C (Fig. 6).This is because several lows in CaCO 3 content coincide with minima in δ 13 C, as is obvious for the B1/B2, PETM and H1/H2 events (Fig. 4).

Calcareous nannofossils
Calcareous nannofossils are generally abundant, diverse, and moderately well preserved.The sole exception is across a 10 cm interval from 28.75 to 28.85 m, which corresponds to the onset of the CIE that marks the PETM.The three samples from this interval are virtually barren of calcareous nannofossils.
Secondary overgrowth of calcite can partially or wholly blur species-specific morphological features.Such diagenetic alteration, however, only marginally influences the relative, semi-quantitative and absolute abundance of calcareous nannofossil taxa (Toffanin et al., 2013).Calcite dissolution, on the other hand, can significantly affect the relative abundances of various calcareous nannofossils within a given volume of sediment.This is because the removal of more dissolution susceptible taxa, such as Toweius and holococcoliths, necessarily increases the abundance of less dissolution susceptible taxa, such as discoasters (Roth and Thierstein, 1972;Adelseck et al., 1973;Roth, 1983;Bornemann and Mutterlose, 2008;Toffanin et al., 2013).In general, moderate to strong calcite dissolution also decreases the total abundance of calcareous nannofossils within a given volume of sediment (Adelseck et al., 1973;Toffanin et al., 2011).In the Cicogna section, calcite overgrowth on discoasters ------------Figure 7. Relative (%) and semi-quantitative (N mm −2 ) abundances of selected calcareous nannofossil genera across the Cicogna section.Also shown are the lithostratigraphy, magnetostratigraphy, biostratigraphy and carbon isotope (δ 13 C) stratigraphy at the Cicogna section (Fig. 4).Color bands and symbols are the same as in previous figures.is the prevalent process affecting calcareous nannofossil assemblages (Plate I).Most assemblages display high abundances (> 1000 specimens mm −2 ) and a high diversity, which include more fragile taxa.It follows that dissolution has not severely altered most assemblages in samples from the Cicogna section.Rather, the calcareous nannofossil record is considered to represent a genuine paleoecological signal.
Nannofossil assemblages from the Cicogna section display several general trends (Figs.7-9).At the most basic level, there is a decrease in the total number of nannofossils (N mm −2 ) with decreasing age.Paleocene samples average approximately 2600 specimens mm −2 , whereas Eocene samples above the H1/H2 events average approximately 1200 specimens mm −2 .This decrease in abundance broadly corresponds to a change in calcareous nannofossil composition, as supported through a series of additional observations at the Cicogna section (Figs.7-9): -Coccolithus and Toweius constitute nearly half of the assemblages considering the entire section.However, these genera show a clear decrease in abundance upsection, with a mean value of 60 % in Paleocene samples and 35 % in Eocene samples.
-Zyghrablithus bijugatus shows a low mean value of approximately 4 % in the Paleocene, followed by a sharp increase in the basal part of the Eocene, and a mean value of approximately 25 % upsection in the Eocene.Hence, the abundance of this taxon expands on behalf of Coccolithus and Toweius.
-Sphenolithus decreases progressively during the Paleocene, suddenly disappears at the onset of the PETM, before returning to and exceeding pre-PETM values in the lower Eocene.Thus, the abundance of sphenoliths also expands on behalf of Coccolithus and Toweius.
-Fasciculithus shows a severe decline in abundance and species diversity at the onset of the PETM (28.70 m), leading up to their extinction at 34.73 m.
-Octolithus is rare throughout most of the studied section, but displays high abundances from approximately 14.7 to 27.5 m in the upper Paleocene.
-Discoaster does not show any distinct change in abundance except for a single peak at the onset of the PETM.
-Several Cretaceous and early Paleocene species constitute minor reworked components throughout the section.Notably, the intervals marked by the PETM, H1/H2 and, to a lesser extent, B1/B2 CIEs are characterized by higher abundances of these reworked components.
-Representatives of placolith genera, such as Prinsius, Ericsonia, Chiasmolithus and Girgisia, are minor components of most samples.Prinsius displays a marked permanent decrease in abundance from a mean value of approximately 6 to 2.5 % across the Paleocene-Eocene boundary.By contrast, Ericsonia does not show a prominent difference in abundance between Paleocene and Eocene assemblages, but increases in abundance during known and suspected hyperthermal events.
-The calcareous nannofossil excursion taxa (CNET), which include Discoaster araneus and the genus Rhomboaster are present during the CIE of the PETM.The evolution of the Rhomboaster/Tribrachiatus plexus started at the onset of the PETM, when Rhomboaster and T. bramlettei first appeared, and continued into the lower Eocene with the successive appearances of T. contortus and T. orthostylus (Raffi et al., 2005;Agnini et al., 2006Agnini et al., , 2007b)).
Beyond the above variations, evolutionary appearances and extinctions occur during the studied time interval (Figs.7-9).Most of these species belong to Discoaster, Sphenolithus and the Rhomboaster/Tribrachiatus lineage, and include D. multiradiatus, D. diastypus, D. lodoensis, S. radians, S. anarrhopus, T. bramlettei, T. contortus and T. orthostylus.The biohorizons defined using these species are exceptionally useful for biostratigraphy and, interestingly, often occur close to changes in δ 13 C.All assemblage data were used for PCA.This indicates that PC1 (41.3 %) and PC2 (14.7 %) together account for 56 % of the variance in the data set.The PCA graph (Figs.10a, S1) shows that samples can be subdivided into three subgroups.The first two populations of samples are distinguished because of their different positions along the x axis (PC1).The third population is much more dispersed but a possible discrimination from the other two seems to be hypothesized because of its different position along the y axis (PC2).The use of a different statistical procedure, such as non-metric MDS, does not substantially change these results (Fig. S2).To further support the subdivision of the study samples in three subgroup, we applied MANOVA to our data set (Fig. 10b).The result clearly confirmed that Paleocene, PETM and Eocene samples are in fact isolated one from each other.

Integrated stratigraphy and a carbon isotope template
Polarity chron boundaries and calcareous nannofossil biohorizons (Table 1; Fig. 4) provide a solid stratigraphic framework for the Cicogna section.Calcareous nannofossil biohorizons, including additional ones defined here, align in same stratigraphic order when compared to other locations, such as ODP Site 1262 and DSDP Site 577 (Table 1; Fig. 11).
Once placed into the above stratigraphic framework, the bulk carbonate δ 13 C profile documented at Cicogna correlates well with that generated at ODP Site 1262 (Fig. 5).In fact, it is similar to δ 13 C profiles generated at multiple locations (Figs. 2, S4), as long as records have been properly calibrated in both the depth and time domains.This includes accounting for core stretching and core gaps at scientific drilling sites, such as at DSDP Site 577 (Dickens and Backman, 2013), and accounting for changing strike and dip along land sections, such as done at Cicogna (Fig. 3).During late Paleocene and early Eocene times, the Cicogna section records the long-term decrease in δ 13 C. Superimposed on this drop were multiple, often paired, negative CIEs.The PETM definitively represents the most prominent CIE, but several other CIEs occurred before and after.Importantly, the relative positions of polarity chron boundaries, key calcareous nannofossil biohorizons and CIEs at Cicogna align well with those found at other locations (Table 1;Figs. 5,11).
A clearly recognizable δ 13 C pattern spans the late Paleocene through the early Eocene at several locations (Cramer et al., 2003;Nicolo et al., 2007;Galeotti et al., 2010;Zachos et al., 2010;Slotnick et al., 2012Slotnick et al., , 2015b)), although the total number of CIEs remains uncertain.At Cicogna, the problem lies in the interval surrounding the K/X event, which broadly corresponds to the start of the EECO (see discussion in Slotnick et al., 2012).We cannot confirm with our sample resolution whether a series of short-term, small-amplitude CIEs mark this time, an idea suggested from δ 13 C records of the Clarence Valley sections (Slotnick et al., 2012(Slotnick et al., , 2015b  ever, as at other locations, such as Site 1262, no significant CIEs occurred within the 1.6 Myr between the PETM and the H-1/ETM-2 event (Fig. 5).
The time-correlative δ 13 C template implies changes in the mean ocean δ 13 C of dissolved inorganic carbon (DIC).In turn, these compositional changes very likely represent variations in fluxes of highly 13 C-depleted carbon to and from the ocean or atmosphere, such as changes in the release and storage of organic carbon (Shackleton, 1986;Dickens et al., 1997;Kurtz et al., 2003;Deconto et al., 2010;Komar et al., 2013).The δ 13 C record at Cicogna offers no direct insight on the location of this carbon (e.g., seafloor methane, permafrost, peat).However, it does support an important concept: the magnitudes of given CIEs appear somewhat related to one another and to the long-term δ 13 C record.In particular, the PETM occurred about halfway between the long-term high and low in δ 13 C, and heralded a relatively long time interval lacking CIEs.A generic explanation is that a very large mass of 13 C-depleted carbon was injected from some organic reservoir into the ocean or atmosphere during the PETM, and that the reservoir needed to recharge for considerable time before the next injection (H-1/ETM-2) could occur (Dickens, 2003;Kurtz et al., 2003;Lunt et al., 2011;Komar et al., 2013).
The overall −1 ‰ offset of the δ 13 C curve between the records at Cicogna and at sites 577 and 1262 (Fig. 5) warrants brief discussion.It probably does not reflect wholesale diagenesis and resetting of the primary signal at any of these sections.Otherwise, a recognizable correlative δ 13 C record and well-preserved nannofossils (Plate I) would not be found at all three locations.In fact, it is difficult to modify the original δ 13 C composition of carbonate over appreciable distance (greater than several meters) in marine sedimentary sequences dominated by fine-grained calcite, even those now exposed on land as lithified rock, such as at Cicogna or in the Clarence Valley.This is because the carbon water / rock ratio remains low, because almost all carbon exists in carbonate, and because temperature minimally influences carbon isotope fractionation (Matter et al., 1977;Scholle and Arthur, 1980;Frank et al., 1999).Instead, the offset in the δ 13 C curves probably relates to differences in the composition of the original carbonate, a concept that we return to later.
However, local dissolution and re-precipitation of carbonate definitely has occurred in the Cicogna section.This can be observed in the overgrowths of secondary calcite on discoasters and Rhomboaster/Tribrachiatus (Plate I).This process should dampen the original CIEs, because on the meter scale, dissolution and re-precipitation of carbonate would involve δ 13 C gradients in the DIC of surrounding pore water (Matter et al., 1977;Scholle and Arthur, 1980).This may explain, in part, why the magnitude of early Paleogene CIEs in bulk carbonate records are often muted relative to those found in other carbon-bearing phases (Slotnick et al., 2015b).

Oxygen isotopes and a problem recording past temperatures
The δ 18 O record at Cicogna is intriguing because many of the CIEs are characterized by negative excursions but absolute values of δ 18 O generally and unexpectedly increase upsection (Fig. 4).Similar results have been documented in bulk carbonate stable isotope records at other locations, such as ODP Site 1215 (Leon-Rodriguez and Dickens, 2010) and Mead Stream (Slotnick et al., 2012).Even the δ 18 O record of bulk carbonate at ODP Site 1262 shows minimal long-term change from the late Paleocene to the early Eocene (Zachos et al., 2010), the time when high-latitude surface temperatures and deep ocean temperatures presumably increased by 5-6 • C, and one might expect a > 1 ‰ decrease in the δ 18 O of marine carbonate.
Like previous workers, we cannot discount the notion that temperatures at low and high latitudes responded differently across the early Paleogene (Pearson et al., 2007;Huber and Caballero, 2011).Unlike for carbon isotopes, however, local dissolution and re-precipitation of carbonate should significantly impact the δ 18 O of marine carbonate.This is because the oxygen water / rock ratio would be high before lithification, and because temperature strongly influences oxygen isotope fractionation (Matter et al., 1975;Scholle and Arthur, 1980;Frank et al., 1999).In general, as calcite-rich sediments and surrounding pore water are buried to higher temperatures along a geothermal gradient, local dissolution and re-precipitation of carbonate shifts carbonate δ 18 O to lower values (above references; Schrag et al., 1995).It is likely that, during sediment burial, the bulk carbonate δ 18 O records in many lower Paleogene sections, including at Cicogna, have been modified.We suggest that a signal of surface ocean temperature changes remains in the Cicogna section, which gives rise to short-term δ 18 O excursions that coincide with CIEs and several known suspected hyperthermal events.However, the entire δ 18 O record at this location likely has shifted to more negative values preferentially with increasing burial depth and age.This partly explains the observed relationship between bulk carbonate δ 13 C and δ 18 O, which lies along a trajectory expected for diagenesis (Fig. 6).A potential test of this idea would be to show that the overgrowths on nannofossils (Plate I) have a significantly lower δ 18 O than the primary core carbonate of nannofossil tests.

Calcareous nannofossil assemblages within the context of correlative stable isotope records
A detailed stable carbon isotope curve provides a powerful tool to place past changes in calcareous nannofossil assemblages into a highly resolved framework.This is because, as implied above, truly global changes in the δ 13 C composition of the ocean should occur within the cycling time of carbon through ocean, which is < 2000 years at present day and presumably for the entire Cenozoic (Broecker and Peng, 1982;Shackleton, 1986;Dickens et al., 1997).
Across the study interval at Cicogna, several calcareous nannofossil taxa appear or disappear (Table 1).Moreover, their abundances also change between these horizons (Figs.7-9).One might hypothesize that these changes in nannofossil assemblages were related to the established (e.g., the PETM, H1/ETM-2 and K/X) and potential (e.g., the B1/B2, I1/I2) hyperthermal events that occurred during the late Paleocene and early Eocene (Figs. 1, 5).However, the timing between recorded evolutionary appearances and extinctions of calcareous nannofossils and perturbations in δ 13 C is variable (Figs.7-9).For instance, several significant calcareous nannofossil changes observed close to H1/H2 hyperthermals (e.g., B T. othostylus, B S. radians, B S. villae, Tc D. multiradiatus, T T. contortus) predate these events.By contrast, several biotic changes observed close to the B1/B2 CIEs (e.g., B D. delicatus, Tc S. anarrhopus, B D. multiradiatus, T Ericsonia robusta) happened at the end of these events.The PETM seems to provide the only case when a negative CIE precisely corresponds to major changes in calcareous nannofossil assemblages.
Profound changes in calcareous nannofossil assemblages occurred across the PETM in several locations (Fig. 2), both in terms of relative abundances and increases in origination and extinction rates (Aubry, 1998;Bown et al., 2004;Raffi et al., 2005;Gibbs et al., 2006a;Agnini et al., 2007a;Self-Trail et al., 2012).At Cicogna, the assemblages show remarkable, though mostly transient, relative abundance variations across the PETM, including an increase in Coccolithus, a decrease in Zygrhablithus, Sphenolithus, Toweius and Prinsius, and an extinction of most fasciculith species (Fig. 8).Not surprisingly, these changes are very similar to those in the Forada section, which is also located in the Belluno Basin (Agnini et al., 2007a).
Although these changes in relative abundance of taxa alone represent a notable difference with respect to background conditions, most of the changes are transient and/or local when compared with other data sets (Bralower, 2002;Gibbs et al., 2006b;Agnini et al., 2007b;Angori et al., 2007;Mutterlose et al., 2007).For instance, an increase in abundance of Discoaster and Fasciculithus was reported for some of the PETM section studied (e.g., Bralower, 2002;Tremolada and Bralower, 2004;Raffi et al., 2009), but these assemblage variations were not observed in other sections (e.g., Gibbs et al., 2006;Agnini et al., 2007a;Self-Trail et al., 2012).The only global calcareous nannofossil assemblage features of the PETM are represented by the evolutionary appearance of Rhomboaster/Tribrachiatus lineage, the presence during the CIE of short-lived species such as Discoaster araneus, and the disappearance of several species of fasciculiths (Raffi et al., 2005;Agnini et al., 2007a).
While changes in calcareous nannoplankton assemblages during the PETM have been investigated at high resolution at different locations (e.g., Bralower, 2002;Gibbs et al., 2006b;Agnini et al., 2007a), the longer-term perspective in which such changes occurred during the early Paleogene has remained uncertain (Gibbs et al., 2012).The record at Cicogna provides this opportunity.
The PCA of calcareous nannofossil census data (%) indicates that two principal components (PC1 and PC2) account for most (56.0 %) of the variability in our 15 selected subgroups.Such analysis also permits the studied samples to be subdivided into two populations and a possible widely dispersed group (Fig. 10a).The first two populations are distinguished because of a major difference along the x axis representing PC1, whereas the third population seems to stand out because of a difference along the y axis representing PC2.Importantly, each of these three populations constitutes a homogeneous group in the time domain: group 1 includes all upper Paleocene samples (Paleocene samples and B1/B2 events); group 2 consists of almost all lower Eocene samples (Eocene samples, H1/H2 events and K event); and group 3 comprises samples that span the PETM (both core and recovery), and two samples that come from sediment deposited during the core of the H1 and B2 events (Fig. 10).These results indicate that late Paleocene calcareous nannofossil assemblages are statistically different in their composition from those of early Eocene samples.To check whether calcareous nannofossil assemblages across the PETM are statistically different from those of either the late Paleocene or the early Eocene, we performed MANOVA, which pointed out that ellipses containing 95 % of the data points for each group (late Paleocene, early Eocene and PETM) are virtually not overlapping one to each other, suggesting that three statistically different populations are recognized across the PETM, the late Paleocene and the early Eocene background assemblages, and the PETM fossil associations.
The general shift in the relative abundance of placoliths (i.e., Coccolithus, Toweius and Prinsius), the major component of the late Paleocene assemblages, to nannoliths/holococcoliths (i.e., Sphenolithus and Zygrhablithus), the major component of the early Eocene assemblages, largely explains the PC1 component or axis 1 (Fig. 10).By contrast, the dramatic shift toward negative values in the PC2 component or axis 2 during the PETM happens because of the increase in Ericsonia and reworking and the presence of Rhomboaster-Tribrachiatus plexus.Presumably, this relates to peculiar paleoenviromental conditions that developed during the event.One can hypothesize that this may have been a major difference in the physicochemical parameters of sea surface waters, such as higher temperatures, higher nutrient concentration or reduced carbonate saturation state.
Statistical analysis of our data from Cicogna does not highlight any prominent short-term changes in calcareous nannofossil assemblages, other than across the PETM and perhaps the B2 and H1 events.However, several biohorizons occur around the B1/B2 events.Specifically, these are the Bc Z. bijugatus, the brief high abundance of spp., the evolutionary onset of the D. delicatus/D.multiradiatus lineage, the presence of the short-ranged E. robusta, the final radiation of late Paleocene fasciculiths (i.e., F. richardii group, F. hayi, F. lilianae, F. alanii), and the Tc of S. anarrhopus.All these happened at Cicogna and at ODP Site 1262 within Chron C25n (Agnini et al., 2007b;Dallanave et al., 2009;Fig. 11), which spanned only 0.54 Myr (Westerhold et al., 2008).These near-synchronous events are intriguing because while the various nannofossils represent only minor components of late Paleogene assemblages, they were destined to become either an abundant constituent of Eocene populations (e.g., Z. bijugatus and the D. delicatus/D.multiradiatus lineage) or extinct after having been a distinctive element of Paleocene assemblages (e.g., Fasciculithus spp.and S. anarrhopus).Following the PCIM, the long-term increase in temperature and decrease in δ 13 C (Fig. 1) coincided with a series of minor changes in nannofossil assemblages, which subsequently became important, presumably for evolutionary reasons.
Similar to the late Paleocene, calcareous nannofossil assemblages after the PETM do not show major rearrangements of common taxa during the early Eocene.Instead, minor components of these assemblages exhibit a sequence of closely spaced biohorizons.The sequence of these biohorizons is T Fasciculithus, B D. diastypus, B T. contortus, T T. bramlettei, Tc D. multiradiatus, T T. contortus, B T. orthostylus, B S. radians, T D. multiradiatus, B D. lodoensis, B G. gammation and Bc D. lodoensis (Table 1).Within the resolution of available paleomagnetic and δ 13 C data, all these biohorizons are virtually synchronous between the Cicogna section and ODP Site 1262 (Fig. 11).They also almost all occurred in near synchrony at DSDP Site 577 (Dickens and Backman, 2013), although the precise correlation remains uncertain, given problems with coring disturbance and subtleties in age models at this location.
Importantly, for stratigraphic purposes, the B and Bc of D. lodoensis are approximately coeval at all three locations and spaced apart by about 750 kyr.Unless one examines samples in detail, these two biohorizons can be confused and result in an erroneous assignment of early Eocene ages.
The evolutionary appearances and extinctions amongst early Eocene nannofossil assemblages may suggest the presence of uneven communities living in an extreme climate in which alterations of environmental conditions, even minor, might trigger evolutionary changes or prominent variations in abundances of a limited number of taxa that typically do not represent the dominant component of assem-blages.A possible explanation is a generally higher tolerance of cosmopolitan taxa to variations in environmental conditions (Boucot, 1975;Winter et al., 1994).In contrast, highly specialized taxa that are adapted to a particular ecological niche may display greater sensitivity to modifications in the photic zone environment (MacArthur and Wilson, 1967;Pianka, 1970;Baumann et al., 2005).
In summary, several genera of calcareous nannofossils, such as Rhomboaster, Tribrachiatus, Sphenolithus, Discoaster and Zygrhablithus were, at least to some extent, affected during the late Paleocene-early Eocene transition, because they show an increased rate of taxonomic evolution (Fig. 11).However, these genera are all minor groups in terms of overall abundance, at least in most lower Paleogene sediment sequences, and they all belong to nannoliths and holococcoliths.It appears that these organisms were more sensitive to environmental changes than heterococcoliths, for example the cosmopolitan genera Coccolithus and Toweius.

Early Paleogene calcareous nannofossil evolution
Any comprehensive paleoenvironmental interpretation involving early Paleogene calcareous nannofossils remains tentative because many taxa, such as Rhomboaster/Tribrachiatus, Discoaster, Sphenolithus and Zygrhablithus, are extinct.Still, some single species or species groups are considered to be useful for reconstructions of paleoenvironmental conditions (Geisen et al., 2004).With that viewpoint, and with an understanding of modern holococcolith/nannolith ecology and classical biogeographical model, we provide a scenario regarding late Paleocene-early Eocene calcareous nannofossil evolution.
Modern holococcolithophores have numerous tiny rhombohedral calcite crystallites, and are considered as haploid stages of certain heterococcolithophores, which can live in just about any marine photic zone environment, although higher abundances and diversity are typical in oligotrophic settings (Billard and Inouye, 2004).The most common Paleogene holococcolith was Zygrhablithus bijugatus.This taxon has been interpreted as a K specialist more adapted to stable environments and oligotrophic conditions (Aubry, 1998;Bralower, 2002;Tremolada and Bralower, 2004;Agnini et al., 2007a;Self-Trail et al., 2012).Nannolith is a term used to describe peculiar morphotypes usually observed in association with coccoliths but lacking the typical features of heterococcoliths or holococcoliths.Ceratolithus cristatus, a modern nannolith, has been observed on the same cell together with Neosphaera coccolithomorpha (Alcolber and Jordan, 1997), suggesting that the nannolith stage (C.cristatus) corresponds to the holococcolith stage in other taxa (Young et al., 2005).Paleogene nannoliths include taxa with peculiar morphologies such as Discoaster, Fasciculithus and Sphenolithus.These genera often have been associated with warm waters and oligotrophic environments and are almost unanimously interpreted as K specialists (Haq and Backman, 1986;Wei and Wise, 1990;Bralower, 2002;Gibbs et al., 2004;2006a, b;Agnini et al., 2007a).K specialists fluctuate at or near the carrying capacity (K) of the environment in which they thrive (MacArthur and Wilson, 1967), and are usually characterized by long individual life cycles and low reproductive potential.The K-specialist strategy is advantageous in highly typically oligotrophic environments, which allows the evolution of stenotopy and where organisms compete by specialization and habitat partitioning (Hallock, 1987;Premoli Silva and Sliter, 1999).The narrow range of adaptability to changes in habitat or ecological conditions stimulates a rapid speciation.At present, it is commonly accepted that modern holococcoliths and nannoliths are not produced by autonomous organisms; rather, they are stages in the life cycle of coccolithophores.Moreover, the passage between the two stages may be triggered by environmental factors (Billard and Inouye, 2004).
Hence, though Paleogene holococcoliths/nannoliths have no direct descendants in present-day oceans, they may very well have shared similar physiological features and life cycles with modern taxa.Assuming this is the case, the increase in the relative abundance of holococcoliths and nannoliths at the expense of heterococcoliths as well as the higher rates of evolution shown by holococcoliths and nannoliths may suggest conditions in which highly specialized taxa could flourish and rapidly evolve.This scenario is consistent with the idea, based on laboratory and modern ocean data, that the calcareous nannoplankton response to environmental change is species-or group-specific rather than homogeneous across the entire assemblage (Riebesell et al., 2000;Langer et al., 2006;Iglesias-Rodriguez et al., 2008;Lohbeck et al., 2012).Variations in the thermal and chemical structure of photic zone waters may thus account for the observed changes in the early Paleogene calcareous nannofossil assemblages.

Carbon isotope of surface waters during the early Paleogene
Like at Cicogna, well-preserved calcareous nannofossils dominate bulk sediment carbonate contents of early Paleogene strata at sites 577 and 1262 (Backman, 1986;Zachos et al., 2004;Dickens and Backman, 2013).Given that the nannofossil assemblages are fairly similar (Fig. 11), a very basic question returns: why is the overall early Paleogene bulk carbonate δ 13 C record at Cicogna less by approximately 1 ‰?A variety of explanations for the δ 13 C offset can be offered.For example, sediments at Cicogna had greater amounts of organic matter, and during burial diagenesis, a fraction of this carbon was consistently added so as to decrease the δ 13 C of pore water DIC.We note, though, that C org contents (wt %) at the proximal Forada section generally have values less than 0.1 wt % (Giusberti et al., 2007).Similar C org contents are found at ODP Site 1262, where values range from 0.0 to 0.3 wt % (Zachos et al., 2004).
A cursory examination of early Paleogene bulk carbonate δ 13 C records from other sites of the North Atlantic/western Tethys region (e.g., sites 550 and 1051; Fig. 2) shows a commonality: these locations also display negative 0.5 to 1 ‰ offsets relative to correlative records at sites 577 and 1262 (Cramer et al., 2003).The δ 13 C of DIC in modern surface waters (< 100 m) ranges by about 2 ‰, because of the differences in temperature, primary productivity and water mass mixing (Kroopnick, 1985;Tagliabue and Bopp, 2008).Notably, however, gradients in δ 13 C of surface water DIC are gradual, such that large regions have fairly similar values.It is possible that bulk carbonate δ 13 C values in early Paleogene North Atlantic sections record lower values than locations near the Equator or in southern latitudes because of past ocean circulation.

Summary and conclusions
We generate records of bulk carbonate δ 13 C and δ 18 O, CaCO 3 content and calcareous nannofossil assemblages from the Cicogna section, a marine sedimentary succession that now crops out along a stream in the Southern Alps of northeast Italy.The combined geochemical and calcareous nannofossil results allow us to generate a detailed stratigraphy for the section, as well as to explore relationships between stable isotope variations and nannofossil assemblages.Most lower Paleogene sections examined to date lack such coupled data sets.
The δ 13 C record and calcareous nannofossil assemblages show that the section spans ∼ 5.3 Myr of the late Paleocene and early Eocene interval, from 57.5 to 52.2 Ma on the WO-1 timescale.This is consistent with previous paleomagnetic information and preliminary calcareous nannofossil biostratigraphy (Dallanave et al., 2009), but provides a more detailed stratigraphic framework, one appropriate for correlations to other locations around the world.In particular, the fairly well resolved δ 13 C record shows long-term and short variations that correspond to those found in several other sections, including an established series of negative CIEs.The most prominent CIE marks the PETM, while other less pronounced CIEs represent the H-1, K/X and other "events" documented elsewhere.The δ 13 C variations observed at Cicogna clearly reflect global changes in the fluxes of carbon to and from the ocean and atmosphere.
PCA of calcareous nannofossil assemblages shows three distinct sample clusters.Late Paleocene and early Eocene assemblages were distinctly different from each other and from that of the PETM.Indeed, the PETM, the most intense hyperthermal during the late Paleocene-early Eocene, was characterized by a unique calcareous nannofossil assemblage composition.This suggests that the brief episode of extreme warming permanently modified the composition of calcareous nannoplankton through an increase in the rate of taxonomic evolution (Gibbs et al., 2006a).Less prominent hy-perthermal events do not show significant variations in the main components of assemblages but rather were characterized by a series of changes affecting a limited number of rare taxa.These taxa may have been less tolerant to environmental changes in their habitats.
More common taxa, essentially consisting of placoliths, such as the cosmopolitan Coccolithus and Toweius, display a progressive long-term decrease by transient changes in their relative abundance but virtually no extinction or origination events occur in these groups during the pertinent time interval.Species belonging to nannoliths and holococcoliths (Discoaster, Fasciculithus, Rhomboaster/Tribrachiatus, Sphenolithus and Zygrhablithus), generally show a higher rate of evolution and a higher concentration of biohorizons close to δ 13 C perturbations.In conclusion, calcareous nannoplankton show a different response of the various components of the assemblages; this is consistent with a species or taxonomic unit sensitivity of calcareous phytoplankton to paleoenviromental perturbations.This evolutionary climate-forced model is supported by data from ODP Site 1262, which demonstrate that these changes are global and synchronous between middle latitudes in the western Tethys region and the South Atlantic.
The Supplement related to this article is available online at doi:10.5194/cp-12-883-2016-supplement.

Figure 3 .
Figure 3.The location and representative photographs of the Cicogna section in northeast Italy.(a) Geographic map showing the main Late Cretaceous-early Paleogene paleogeographic domains of the Italian Southern Alps (modified after Cati et al., 1989); (b) geological map of the local area (modified after Costa et al., 1996, indicating also the location of the Cicogna section; red asterisk); (c) alternating beds of Paleocene gray-green marls and calcareous marls (0-20 m); (d) the Scaglia Rossa sensu lato overlain by the Belluno Flysch; (e) marl/calcareous marl couplets in the lower Eocene portion of the section (approximately 40.0-70.0m); (f) the base of the Clay Marl Unit, which denotes the onset of the PETM (approximately 28.7-29.3m); and (g) the brownish-red interval of clayey marls with sporadic gray-green centimeter-scale spots and lenses, the CMU, overlain by prominent rhythmic alternations of marls and calcareous marls (approximately 28.7-33.0m).

Figure 4 .
Figure 4.The Cicogna section with records of bulk carbonate δ 13 C and δ 18 O data, and CaCO 3 content.Calcareous nannofossil biostratigraphy (CP and NP biozones) and magnetostratigraphy are afterDallanave et al. (2009); CN biozones are also reported.Orange and yellow bands mark major δ 13 C excursions.Dashed lines indicate minor CIEs that have been labeled elsewhere (e.g., E1/E2, F and G;Cramer et al., 2003), whereas dotted lines indicate minor changes in δ 13 C that appear to occur also at ODP Site 1262 (see also Fig.5).

Figure 5 .
Figure 5. Stratigraphic correlation between upper Paleocene and lower Eocene sections at Cicogna, ODP Site 1262(Zachos et al., 2010), and DSDP Site 577(Cramer et al., 2003;Dickens and Backman, 2013).All three sites have independently derived nannofossil biohorizons, polarity chrons and δ 13 C records, which account for subtle temporal offsets.Color bands and symbols are the same as in Fig.4.Note the missing record at Site 577 that corresponds to known core gaps.

Figure 6 .
Figure6.Plots of (a) bulk carbonate δ 13 C versus bulk oxygen δ 18 O and (b) bulk carbonate δ 13 C vs. CaCO 3 content for samples from the Cicogna section.The black arrow shows the expected effect of burial diagenesis.Grey and black dash lines are linear trend lines for Paleocene and Eocene samples, respectively.Note the clear distinction in δ 13 C for Paleocene and Eocene samples, which relates to a long-term decrease in δ 13 C (Fig.5).

Figure 8 .
Figure 8. Relative (%) and semi-quantitative (N mm −2 ) abundances of selected, mainly late Paleocene, calcareous nannofossil taxa across the Cicogna section.Also shown are the lithostratigraphy, magnetostratigraphy, biostratigraphy and carbon isotope (δ 13 C) stratigraphy at the Cicogna section (Fig. 4).Color bands and symbols are the same as in previous figures.

Figure 9 .
Figure9.Relative (%) and semi-quantitative (N mm −2 ) abundances of selected, mainly early Eocene, calcareous nannofossil taxa across the Cicogna section.Also shown are the lithostratigraphy, magnetostratigraphy, biostratigraphy and carbon isotope (δ 13 C) stratigraphy at the Cicogna section (Fig.4).Color bands and symbols are the same as in previous figures.

Figure 10 .
Figure 10.Statistical Analyses of calcareous nannofossil percentage data of the Cicogna section.Calcareous nannofossils are subdivided into 15 subgroups (Chiasmolithus, Coccolithus, Ellipsolithus, Discoaster, Ericsonia, Fasciculithus, Girgisia, Octolithus, Prinsius, Sphenolithus, Toweius, Rhomboaster/Tribrachiatus, Zyghrablithus, reworking, others).(a) Principal component analysis (PCA) scatter plot of percentage data of calcareous nannofossil taxa of samples from the Cicogna section in terms of the first and second component.Each sample is represented by a circle and labeled.Green and blue shaded areas are the ellipses containing 95 % of the data points of Paleocene group and Eocene group, respectively.(b) Multivariate analysis of variance (MANOVA).Scatter graph and biplot.Each sample is represented by a black symbol (quadrangle and diamond) and labeled.Green, red and blue shaded areas are the ellipses containing 95 % of the data points of Paleocene group, PETM group and Eocene group, respectively.

Figure 11 .
Figure 11.Comparison of δ 13 C profiles and semi-quantitative abundance patterns of selected calcareous nannofossil taxa from the Cicogna section and ODP Site 1262.Calcareous nannofossil biohorizons from DSDP Site 577 are reported in the right part of the figure.Orange and yellow bands mark CIEs shown in previous figures.Color bands and symbols are the same as in previous figures. ).