Nannofossil imprints across the Paleocene-Eocene Thermal Maximum

ABSTRACT

Imprint fossils of nannoplankton preserved on organic matter provide a tool to test the degree and timing of CaCO3 dissolution throughout intervals where CaCO3 preservation is poor (Slater et al., 2022).Although other approaches have been applied to PETM strata to understand the impact of dissolution, such as foraminiferal fragmentation and dissolution of nannofossil rims and central areas (Bralower et al., 2014), these methods rely on the preservation of CaCO3 and are not necessarily indicative of the timing of dissolution.For example, dissolution of nannofossil liths could occur at any point after their formation; in the water column, at the seafloor, or after deposition and lithification.
Nannofossil imprints, however, can be preserved in sediments devoid of CaCO3, and where this is the case, they can reveal that CaCO3 has been removed from the rock record after deposition (Slater et al., 2022).
Here we searched for nannofossil imprints through a PETM succession from the South Dover Bridge (SDB) core, southern Maryland, USA [38°44′49.34″N,76°00′25.09″W](Fig. 1; drilled by the U.S. Geological Survey), with the aim to determine the timing of potential CaCO3 dissolution.The SDB section was chosen because it represents a relatively shallow water marine environment (~120-(Alemán González et al., 2012;Edwards 2012), required for imprint preservation (Batten 1985;Slater et al., 2022).Furthermore, the succession appears to record a spectrum of dissolution conditions through the PETM interval, from little to no dissolution below and above the CIE, to pervasive dissolution at the base of the carbon isotope excursion (CIE).The calcareous nannoplankton 'body' fossils (i.e., the calcite fossil remains of nannoplankton cell wall coverings) from this succession have previously been studied in detail, with diverse and abundant assemblages spanning the PETM described by Self-Trail (2011), Alemán González et al. (2012) and Self-Trail et al., (2012).A notable ~2-m-thick dissolution zone has been recognized near the base of the CIE, through which nannoplankton body fossils are extremely sparse (Self-Trail 2011;Self-Trail et al., 2012).Bralower et al. (2018) described a low carbonate interval (LCI) -representing a slightly amended version of the dissolution zone -from several PETM sections across Maryland and New Jersey, including the SDB core.Bralower et al. (2018) discussed numerous possible causes for the LCI, hypothesizing that this was likely due to shoaling of the lysocline and calcite compensation depth (CCD), but that eutrophication and microbial activity potentially exacerbated the impact of acidification.Further proxy-based reconstructions of seawater pH from the SDB core have inferred that OA started prior to the main CIE, during a pre-onset excursion (Babila et al., 2022).Indeed, these studies point to relatively shallow-water OA.However, rich and abundant nannofossil imprints preserved within the sediments low in CaCO3 could reveal that CaCO3 was removed by diagenetic dissolution, rather than in situ water column OA or changes to the CCD or lysocline depth that affected seafloor carbonate.

Such results would not necessarily discount that changes to seawater chemistry influenced CaCO3
PETM records, but could provide an indication of the extent of diagenetic CaCO3 dissolution.
Post-drilling dissolution of carbonate is common in organic-rich sediments of the Atlantic Coastal Plain, likely due to pyrite oxidation, thus sampling for body fossils needs to occur as soon as possible after coring (Self-Trail & Seefelt 2005;Self-Trail 2011).This is likely why the sediments of the Marlboro Clay in the SDB core record abundant body fossils, whereas their outcrop counterparts are generally barren or yield very sparse nannofossils (Bybell & Gibson 1991;Bybell & Gibson 1994;Gibson & Bybell 1994;Self-Trail 2011).As the SDB core was recovered in 2007, it is probable that at least some post-drilling dissolution of CaCO3 has taken place; a secondary goal of this study was therefore to examine the nannofossil assemblages using an approach that may be immune to the modifying effects of diagenetic and post-drilling dissolution, by studying their imprints.

METHODS
We examined 12 samples spanning the PETM of SDB (Fig. 2).Rock samples were dissolved in HCl and HF and resultant residues were sieved at 5 µm to isolate organic matter.Processing was conducted at Global Geolab Limited, Canada.Final residues were studied using light microscopy (LM) with an Olympus BX53 and scanning electron microscopy (SEM) using an ESEM FEI Quanta FEG 650 scanning electron microscope at the Swedish Museum of Natural History.
For LM, residues were strewn across cover slips and mounted onto glass slides with epoxy resin.To assess the composition of organic matter, palynofacies analysis was conducted, where a minimum of 300 organic particles were counted per sample (see Table S1 for palynofacies categories).
For SEM, residues were strewn across SEM stubs, dried and gold coated.Organic matter on SEM stubs was observed in systematic traverses for 2 hours per sample at ×10,000 magnification, followed by 30 minutes at ×5,000 magnification, through which all potential imprints were photographed; followed by 30 minutes at ×5,000 magnification to search for well-preserved specimens.Unprocessed rock material from two samples (PJ-SDB13-003 and PJ-SDB13-004) was also examined for imprints and body fossils.For this approach, freshly cleaved rock was mounted onto SEM stubs, gold coated and examined for 2 hours per sample at ×10,000 magnification.

Nannofossil Imprints vs. Body Fossils
Imprint assemblages were considerably less rich than previously sampled body fossils, demonstrating that body fossils capture a more complete record of nannoplankton through the PETM of SDB (Fig. 3).Previous studies have shown that body fossils are extremely sparse through the dissolution zone/LCI (Fig. 3; Self-Trail 2011;Self-Trail et al., 2012;Bralower et al., 2018).
Observations of rock surfaces and organic residues from the sample taken from the dissolution zone/LCI here (PJ-SDB13-003), reveal similar findings; one taxon, either Braarudosphaera sp. or Micrantholithus sp.(a more definitive identification was difficult since only a side-view was visible), was recorded on rock surfaces (Figure S1), and no imprints were found in organic residues.For sample PJ-SDB13-004, which yielded the richest imprint assemblage, body fossils on rock surfaces were common and well-preserved (Figure S1).

Organic Matter
Palynofacies assemblages were co-dominated by phytoclasts, amorphous organic matter (AOM) and marine palynomorphs.The dinoflagellate, Apectodinium, was present through the PETM, recording the acme interval associated with this event (Bujak and Brinkhuis 1998;Crouch et al., 2001).Amorphous organic matter increases in relative abundance around the onset of the CIE, within the dissolution zone/LCI, reflecting a relative increase in organic matter deposition, and a corresponding decline in CaCO3 preservation, associated with the PETM (Zachos et al., 2005; Schneider-Mor & Bowen 2013).

DISCUSSION
The rarity of nannofossil imprints across the PETM suggests that the taphonomic conditions required for their preservation were sub-optimal compared to body fossils (Fig. 3; Self-Trail 2011; Self-Trail et al., 2012).Imprints were only recorded from strata that also yielded body fossils (Fig. 3) and none were found on unprocessed rock surfaces.Hence, rather than representing 'ghost' nannofossils -imprints found in rocks that are barren of body fossils (Slater et al., 2022) -imprints here are likely the molds of body fossils that were dissolved during acid digestion in the laboratory.
Although only one sample was examined from the dissolution zone/LCI here, both the absence of imprints and rarity of body fossils suggests that: (i) nannoplankton production declined through the early stages of the PETM; and/or (ii) dissolution of CaCO3 occurred before lithification, in the water column, at the seafloor, or during the earliest stages of diagenesis.If dissolution occurred after lithification, we would expect to find imprints, as overburden would have likely facilitated their formation.At this stage, our data alone cannot discount interpretations (i) or (ii), but previously studied nannoplankton counts with taxon-specific Sr/Ca data from other localities support the hypothesis that the decrease in CaCO3 through the PETM was primarily driven by an increase in seafloor dissolution, rather than a decrease in production in surface waters (Gibbs et al., 2010).The scarcity of imprints suggests that the timing of potential CaCO3 dissolution was unlikely to have been post-lithification and our findings therefore support the hypothesis that shelf acidification linked to shoaling of the lysocline and CCD contributed to the decline in CaCO3 preservation at the onset of the PETM in the SDB region (Bralower et al., 2018).
Nannofossil imprint assemblages from Mesozoic oceanic anoxic events (OAEs), and especially the Toarcian-OAE, are generally richer, more numerous and better preserved than those documented here (Slater et al., 2022).In addition to variations in seawater chemistry, these discrepancies likely also relate to the amount and type of organic matter -and in particular the quantity of AOM, since this is a good substrate for imprinting (Slater et al., 2022) -preserved through these different events and localities.Given that organic matter appears necessary for imprinting, the rarity of imprints through the PETM compared to the OAEs is likely a product of the lower relative abundances of AOM and the generally lower total organic carbon values (Bralower et al., 2018) compared to the OAEs (e.g., McArthur et al. 2008).Furthermore, the preservation of AOM as larger fragments through the OAEs (Slater et al., 2022) appears to be important, because imprints are less distinct on the smaller, highly fragmented pieces that are typical of the PETM of SDB.Although imprints are apparently most common on AOM compared to other types of organic matter, they can also preserve on dinoflagellates (Downie 1956), prasinophyte algae, and pollen (Slater et al., 2022).The lack of imprints on the dinoflagellate Apectodinium, which is abundant through the PETM of SDB, suggests that the surface of this cyst was a poor substrate for imprinting.
Comparisons of imprint and body nannofossil records through the Mesozoic OAEs revealed marked differences in abundance and diversity patterns between these fossil records.In numerous OAE samples, imprint assemblages were substantially more diverse than body fossil records, and in many cases, rich imprint records were found in samples barren of body fossils (Slater et al., 2022).This is not the case for the PETM of SDB.Although the sampling resolution of imprints here is lower than body fossil records (Self-Trail 2011;Self-Trail et al., 2012), the pattern of lower imprint richness through the studied succession is consistent.More generally, the abundance of body fossils and the scarcity of imprints throughout most of the PETM of SDB indicates that post-burial CaCO3 dissolution was less pervasive compared to the OAE records.These observations bolster confidence that traditionally-sampled body fossil records (Self-Trail 2011;Self-Trail et al., 2012) have not been extensively modified by post-burial dissolution and thus provide a relatively good representative signal of the buried CaCO3 in the Atlantic Coastal Plain region.

CONCLUSIONS
Imprint fossils of nannoplankton represent a relatively novel tool with which to test the extent and timing of CaCO3 dissolution through geological intervals where CaCO3 preservation is poor.In the case of the PETM, the scarcity of these fossils through intervals of low CaCO3 preservation suggests that any dissolution to have happened took place before lithification, in the water column or at the seafloor, supporting hypotheses of seafloor and/or potentially shallower-water CaCO3 dissolution.Future studies testing for the presence and abundance of nannofossil imprints through the PETM at higher-resolution, and in deep-water successions elsewhere, will potentially shed more light on the timing of dissolution through this important geological interval.

FIGURE CAPTIONS Figure 1 .
FIGURE CAPTIONS

Figure 2 .
Figure 2. Selected nannofossil imprints on organic matter.A, imprints on AOM, sample PJ-

Figure S1 .
Figure S1.Scanning electron micrographs of selected nannoplankton body fossils on rock