Assessing Environmental Change Associated with Early Eocene Hyperthermals in the Atlantic Coastal Plain, USA

. Eocene transient global warming events (hyperthermals) can provide insight into a future, warmer world. While much research has focused on the Paleocene-Eocene Thermal Maximum (PETM), hyperthermals of smaller magnitude can be 25 used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, Eocene Thermal Maximum 2 (ETM2 and H2) for the first time in a shallow marine setting along the United States Atlantic margin in the Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil and dinocyst biostratigraphy, and recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite. We assess local environmental change in the Salisbury Embayment utilizing clay mineralogy, marine palynology, δ 18 O of biogenic calcite, and biomarker 30 paleothermometry (TEX 86 ). Paleo-temperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of warming does not correspond to the CIE of ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors other than pCO 2 forcing influenced temperature changes in the region. The ETM2 interval exhibits a shift in clay mineralogy from smectite-dominated facies to illite-rich facies, suggesting hydroclimatic changes but with a rather dampened weathering response relative to that of the PETM in the same 35 region. Organic walled dinoflagellate cyst assemblages show large fluctuations throughout the studied section, none of which seem systematically related to ETM2. These observations are contrary to the typical tight correspondence between climate change and assemblages across the PETM, regionally and globally, and ETM2 in the Arctic Ocean. The data do indicate very


Introduction
The Early Eocene is punctuated by a series of transient phases of carbon input and global warming, termed hyperthermals (e.g., Thomas and Zachos, 2000;Cramer et al., 2003;Littler et al., 2014).The well-studied Paleocene-Eocene Thermal Maximum (PETM) may represent the closest approximation to modern day warming in the geologic record (Zachos et al., 45 2008).However, debate exists as to the origins of this event with respect to the primary source of carbon (Zeebe and Lourens, 2019;Zachos et al., 2010;Reynolds et al., 2017;Dickens et al., 1997;Frieling et al., 2019;Gutjahr et al., 2017).The mass of C released and rise in pCO2 is relatively well constrained by records of seafloor carbonate dissolution and surface ocean pH (Zachos et al., 2005;Penman et al., 2014;Gutjahr et al., 2017).Along continental margins, for example in the Salisbury Embayment, located within the Mid-Atlantic Coastal Plain of the United States, the PETM is marked by sedimentological and 50 biotic evidence of strong hydrological perturbations (Kopp et al., 2009;Sluijs and Brinkhuis, 2009;Stassen et al., 2012b;Self-Trail et al., 2012;Self-Trail et al., 2017b;Rush et al., 2021).Eocene Thermal Maximum 2 (ETM2), ~2 million years after the PETM is characterized by warming and C release, though each about 1/3 that of the PETM (Lourens et al., 2005;Sluijs et al., 2009;Stap et al., 2010;Dunkley Jones et al., 2013;Gutjahr et al., 2017;Harper et al., 2020).ETM2 can provide insight into the climatic impacts of a smaller carbon forcing (pCO2) when compared to the PETM from geographically similar sites, and 55 hence sensitivity to greenhouse forcing.
Intensification of the hydrologic cycle is a critical feature of the climatic response to greenhouse forcing (e.g., Carmichael et al., 2017).Evidence for a pronounced mode shift during the PETM includes enhanced sediment-fluxes in sections from the mid-Atlantic margin which suggest increased frequency of precipitation extremes leading to high energy fluvial activity as well as associated abundances of low-salinity tolerant biota (John et al., 2008;Self-Trail et al., 2017b;Stassen et al., 2012b;60 Rush et al. 2021;Sluijs and Brinkhuis, 2009).Similar signals have been found in many regions across various hyperthermals, including the Pyrenees, the margins of the Tethys, New Zealand, the North Sea Basin, and the arctic (Schmitz and Pujalte, 2007;Jiang et al., 2021;Slotnick et al., 2012;Sluijs et al., 2014;Sluijs et al., 2007;Jin et al., 2022;Harding et al., 2011).
These changes in extreme precipitation patterns are consistent with projections for future change (Pfahl et al., 2017;Swain et al., 2018).Studying the impacts of hydroclimatic changes in response to differing degrees of carbon forcings can inform as to 65 the linearity of these responses, "tipping point" thresholds, and background variability of the hydrosphere.
Given the extensive evidence of a mode shift in coastal mid-Atlantic hydroclimate during the PETM, we initiated a regional search for ETM2.We here identify ETM2 and H2 (~54 Ma) in the Knapps Narrows core based on biostratigraphic, paleomagnetic, and chemostratigraphic data.We present proxy records of the regional climatic response, including https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.foraminiferal δ 18 O and TEX86 based temperature reconstructions, clay mineralogy, and organic walled dinoflagellate cyst 70 ('dinocyst') assemblages and compare the results to published records of the PETM from this region.
2 Materials and Methods

Material
The Knapps Narrows core was drilled at 38.72129N, -76.33162W on the Eastern Shore of Maryland in close proximity to drill sites which have been extensively studied for the PETM, South Dover Bridge and Howards Tract (Figure 1).The cored target 75 interval lies between 84-102 meters in the Nanjemoy Formation.From the bottom of the section to 88.4 meters, lithology is dominated by very coarse-to fine-grained, poorly sorted, angular to subrounded sand, comprised of ~50% glauconite, ~30% quartz, and <5% muddy matrix.Between 88.4 meters to the top of the section, the core largely consists of very coarse to medium-grained, subrounded to angular clayey sand with poorly sorted glauconite (~40%) and quartz (~10%) in a muddy matrix (20%).Sand fines upsection and clay content decreases.The sediments are heavily bioturbated throughout.Due to local 80 faulting, the Marlboro Clay deposited during the Early Eocene PETM is entirely absent and an expanded slightly younger Lower Eocene interval equivalent to calcareous nannofossil zone NP11, which contains ETM2, is present (Cramer et al., 2003;Lourens et al., 2005).This disconformity and resultant paleobathymetry likely contributed to the enhanced sedimentation rates and expanded section recorded in the core (Appendix 1).Initial core description noted much coarser grain-sizes when compared to the Nanjemoy in nearby cores, further supporting the interpretation of rapid infilling during this time.All raw 85 data are included in the supplemental datasets that are posted at PANGAEA.

Nannofossil Biostratigraphy
The calcareous nannofossil biozonation was established and core descriptions were made in the field and at the U.S. Geological Survey (USGS) Florence Bascom Geoscience Center.Forty-two samples for calcareous nannofossil analysis were taken from 90 the center of freshly broken core to avoid contamination from drilling fluid.Smear slides for calcareous nannofossil analysis were prepared using the standard technique of Bown and Young (1998) combined with the double slurry technique of Blair and Watkins (2009) and mounted using Norland Optical Adhesive 61.Slides were examined under cross-polarized light using a Zeiss Axioplan 2 light microscope (LM) at 1250x magnification.Biostratigraphic zones assigned to each species were based on the NP zonation of Martini (1971) and supplemented by the biohorizons of Agnini et al. (2014).Smear slides are housed in 95 the USGS calcareous nannofossil laboratory in Reston, VA.

Paleomagnetism
Paleomagnetic data were generated at the Paleomagnetism Lab of Lehigh University.A total of ~100 minicores were collected from the Knapps Narrows core (40-110 m) with sampling spacing of 0.2 to 1.5 m guided by preliminary biostratigraphic constraints.Stepwise thermal demagnetization in an ASC Model TD 48-SC thermal demagnetizer was applied to the samples 100 followed by measurement on a three axis 2G Enterprises 755 superconducting rock magnetometer.Heating involved at least 7 treatments, at ~25 °C/50 °C steps from 100 °C to 350 °C for the weak samples and up to 575-600 °C for stronger ones.
Thermal demagnetization generally ceased when the remanent magnetization displayed either anomalous surges in magnetization, was too weak for magnetometer precision, or exhibited irregular magnetic directions/intensities for two or three consecutive steps.The choice of a thermal rather than an AF-demagnetization procedure was due to potential existence of 105 high-coercivity magnetic minerals (e.g., goethite).
Samples from rotary cores have no control on their declination orientation.The overprint components from drilling-induced magnetization and from normal-polarity present-day (Late Quaternary) field remanence are observed and could often be easily removed during initial demagnetization steps.Instead, normal (N) or reversed (R) polarity could be indicated by natural remnant magnetization (NRM), pointing downward (N) or upward (R).The present-day magnetic field direction is -11.11°110 declination and 64.88° inclination (from https://www.ngdc.noaa.gov/geomag-web/#igrfwmmfor the modern Knapps Narrows core location), which should be roughly the same as the Eocene since no major continental movement has occurred.
Polarity zones were assigned to stratigraphic clusters of good quality characteristic remanent magnetization (ChRM) vectors, while uncertain intervals were assigned to clusters of noisy magnetic behaviors and to significant gaps in sampling coverage.
Detailed paleomagnetic data and additional data analysis protocol are available in the supplementary paleomagnetism excel 115 workbook.
Given that sediments are largely semi-consolidated, a non-magnetic plastic tube was inserted in the core to extract a sediment plug.The plug was then extruded into aluminum foil pieces and tightly wrapped.Occasionally, paleomagnetic samples were taken by pushing non-magnetic cubes directly into the center of the core halves so that alternate field demagnetization could be used.120 The interpreted polarity and characteristic directions of the samples were given a quality rating of 'N(R)', 'NP(RP)', 'NPP(RPP)', 'N?(R?)' or 'INT' according to a semi-subjective judgment of the behavior of the magnetic vectors during the stepwise demagnetization.The 'N/R' ratings were generally assigned to those samples that attained a (semi-)stable endpoint direction (Characteristic remanent magnetization, or ChRM) during progressive demagnetization.However, only a small subset of our samples displayed this ideal resolution of primary magnetization.We applied a 'P' tag of 'RP' or 'NP' when the 125 residual magnetization vector was considered close to attaining an endpoint before losing its residual magnetization or experiencing a surge.The 'PP' tag was applied to samples that had a distinct trend toward the polarity hemisphere to be https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.considered indicative of the underlying polarity but were considered to be too far from attaining an endpoint before dying to be used in statistics for computing a mean direction.The '?' qualifier was used to denote possible trends toward an underlying polarity, and 'INT' is either entirely uncertain or displayed an endpoint that was intermediate between the 'N' and 'R' poles.130 Polarity zones were assigned to stratigraphic clusters of ChRM vectors rated as N-NP-NPP or R-RP-RPP.Uncertain intervals were assigned to clusters of N?-INT-R?and to significant gaps in sampling coverage.Summary plots of the lithology of the Knapps Narrows with the interpreted polarity patterns (Appendix 2) were generated with the public TSCreator database and visualization software (https://timescalecreator.org).

Bulk isotope stratigraphy and age model 135
Bulk carbonate content and bulk carbonate and benthic foraminiferal stable carbon and oxygen isotope analysis was performed in the Earth and Planetary Sciences Department at the University of California, Santa Cruz.CaCO3 content was determined using a UIC CM140 coulometer.In this system approximately 20 mg of sediment is dissolved in 2 N sulfuric acid.For isotope analyses of bulk carbonate, a mass of ground sediment was weighed to obtain 35 and 50 μg of carbonate.All samples were analyzed with a Kiel IV Carbonate Device paired with a Thermo Scientific MAT253 Mass Spectrometer.The analytical 140 precision of this system based on replicate analyses of Carrera marble standards is better than 0.05‰ for carbon isotopes.
An age model was constructed for this section on the basis of carbon isotope stratigraphy, specifically the carbon isotope excursion (CIE) minima and maxima (respectively H1 and H2) correlated to the orbitally tuned deep ocean records of Lauretano et al. (2015).This age model was then used to calculate sedimentation rates.

Spectral analysis 145
Spectral analysis (eCOCO) of the gamma ray wireline log of the cored interval was performed via Acycle v2.4.1 to obtain an estimate of the sedimentation rate variations (Li et al., 2019).The eCOCO analysis computes the correlation coefficient (COCO, ρ) between the frequency spectra of target astronomical solutions and the presence of different cycles within studied data, which provides the numerical estimation of the depositional rates (higher correlation coefficient corresponds to the most likely sedimentation rates) within a sedimentary record (Li et al., 2018(Li et al., , 2019)).Monte Carlo simulations are embedded in the 150 COCO method to test the significant level of astronomical forcing assumption.

Palynology
Eighteen samples were processed at Utrecht University for quantitative palynological analysis.Between 3 and 15 g of freezedried, lightly crushed sediment was spiked with a known amount of exotic Lycopodium spores and subsequently treated with 30% HCl and twice with ~38-40% HF to dissolve carbonates and silicates, respectively.The insoluable residue was sieved 155 using 15 and 250 μm nylon mesh sieves, with ultrasonic bath steps to break up agglutinated organic matter.The resulting 15-https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.palynomorph categories (pollen, spores, aquatic algae) and dinocysts in particular, was performed with light microscopy, counting at least 200, and typically >250 elements.For taxonomy, we refer to that cited in Williams et al. (2017), with the exception of Wetzelielloid taxa (see Bijl et al., 2017).For paleoecological reconstructions, dinocyst taxa are grouped in 160 complexes of morphologically related species and genera with assumed similar paleoecological affinities following the empirically-based complexes of e.g., Frieling and Sluijs (2018).

Clay Mineral Analysis
Clay mineral analysis was performed in the Earth and Planetary Sciences Department at the University of California, Santa Cruz.Methods were adapted from Kemp et al. (2016), Gibson et al. (2000), and Poppe et al. (2001).Samples were gently 165 crushed and placed in a 1% sodium hexametaphosphate (Calgon) solution adjusted to a pH of 7-7.5 via the addition of ammonium hydroxide.Samples were disaggregated by placing them on a shaker table at approximately 200 rpm for 48 hours.
Samples were then wet washed through a 63 μm sieve to separate the coarse and fine fraction.Samples were then marked 5 cm from the surface of the solution and shaken to resuspend the clay minerals.The suspension was allowed to settle for 4 hours and 6 minutes at 20°C in accordance with Stoke's Law.The suspension above the 5 cm mark was then extracted via 170 syringe to isolate the <2μm size fraction.This suspension was then dried and approximately 150 mg of this clay-sized fraction was then resuspended in deionized water.The suspension was then passed through a sub-micron filter attached to a vacuum apparatus to remove the water and orient the clays.The filter paper was removed and placed on the side of a glass beaker before being transferred to a glass slide to produce an oriented mount.For the glycolation step, mounts were placed inside a desiccation chamber with ethylene glycol at 60°C for a minimum of 4 hours.175 Oriented mounts were scanned in a Philips 3040/60 X'pert Pro X-ray diffraction unit from 0-35° 2θ with a 1-degree source beam and a 1/16 degree receiving slit at 45 mA and 40 mV.The results were then printed, and individual sample peaks were weighed to provide a semi-quantitative measurement of clay mineral abundances.

Foraminifer Oxygen Isotope Paleothermometry
Temperatures based on benthic foraminiferal oxygen isotopes ratios were estimated following protocol outlined in Hollis et 180 al. (2019).The paleotemperature equation of Marchitto et al. (2014) was used.Four to six Anomalinoides acutus specimens were picked from the 180-250 μm size fraction at each interval.All samples were analyzed with a Kiel IV Carbonate Device paired with a Thermo Scientific MAT253 Mass Spectrometer.The analytical precision of this system based on replicate analyses of Carrera marble standards is better than 0.05‰ and 0.10‰ for δ 13 C and δ 18 O, respectively.In order to calculate an absolute temperature, bottom-water δ 18 OSW was estimated based on the relationship between salinity and δ 18 O established by 185 Fairbanks (1982) and the modern-day salinity measurements reported in Richaud et al. (2016), with a -1.2‰ correction applied to account for the lack of ice during the Early Eocene.

TEX86 biomarker paleothermometry
TEX86 analysis was performed on 18 samples at Utrecht University.Glycerol dialkyl glycerol tetraether (GDGT) lipids were extracted from ~10 g of powdered, freeze-dried sediment in 25 ml solvent mixture of dicholormethane (DCM):methanol 190 (MeOH) (9:1, v/v) by a Milestone Ethos X Microwave Extraction System, set to 70 °C for 50 minutes.Filtered (using NaSO4 column) total lipid extracts were dried and separated in apolar, neutral and polar fractions through AlOx column chromatography, with hexane/DCM (9:1), hexane/DCM (1:1) and 1:1 DCM/MeOH (1:1), respectively as mobile phases, dried again and weighed.Polar fractions, after addition of a GDGT standard (99 ng of m/z 744) for quantitative analyses, were diluted in hexane/isopropanol (99:1) to a concentration of 2 mg/ml.A 10 μl aliquot was filtered (0.45 μm 195 polytetrafluoroethylene) and analyzed by high-performance liquid chromatography (HPLC) coupled to ionization mass spectrometer.Analytical precision was 0.006 TEX86 units, determined using an in-house GDGT standard.We use several indices to constrain potential confounding effects such as GDGT contributions from methanogenic/methanotrophic microorganisms, terrestrial sources and deepwater archaea.TEX86 values were calculated from isoprenoid GDGT abundances following Schouten et al. (2002) and converted to sea surface temperature (SST) using linear and exponential calibrations (e.g., 200 Kim et al., 2010;O'Brien et al., 2017), following recommendations by Hollis et al. (2019).Calibration errors are in the order of ±2.5°C (1 SD), but any offset from this calibration is assumed to be a constant throughout the study section.

Stratigraphic Framework
A distinct magnetic reversal pattern is preserved in the Knapps Narrows core with the section below 99.21 m (sample KN37) 205 exhibiting a reversed polarity and the upper section (above 91 m, sample KN36) a normal polarity.The reversal boundary, identified as the transition from magnetochron C24r to C24n based on nannoplankton stratigraphic data, thus lies within 99.21-91 m (Figure 2).According to nannoplankton data, the lowermost section is placed in lower Zone NP10 based on the first occurrence (FO) of Rhomboaster spp.and the absence of Tribrachiatus contortus.Upper Zone NP10 is identified at 96.6 m by the FO of T. contortus and basal Zone NP11 is identified at 95.1 m by the last occurrence of T. contortus along with the 210 presence of T. orthostylus.This reconstructed biomagnetostratigraphy (Appendix 2, 3) is generally consistent with the Paleogene time scale 2020 (Speijer et al., 2020) and timing of ETM2 (Cramer et al., 2003;Lourens et al., 2005;Westerhold et al., 2007;Stassen et al., 2012a).
The COCO and eCOCO (Figure 4) analysis shows an optimal sedimentation rate of ~5 cm/kyr for most of the succession from 103.5 m to 43.5 m except the interval of 90 m to ~82 m, where there is a transition to a sedimentation rate of approximately 13 cm/kyr.Assuming orbital forcing, the ~5 cm/kyr, and a cycle of ~18 m (Supplementary Figure 5) correlates to the 405 kyr long eccentricity period.Given the surge in estimated sedimentation rate, then the observed ~18 m/cycle should instead 225 correlate to the short eccentricity periodicity.Given the sudden increase in deposition rate, this interval may reflect on other factors, for example a period of enhanced precipitation even though this period of elevated sedimentation rates does not correspond to any hyperthermal event, or bathymetric changes related to regional faulting.

Palynology
Palynological associations are dominated by marine palynomorphs, notably dinocysts and common to abundant organic linings 230 of benthic foraminifera.Terrestrial palynomorphs such as pollen and spores are rare.Dinocyst assemblages are well preserved and diverse.They comprise a typical mid Early Eocene shelf assemblage given abundant contributions of Spiniferites spp., Operculodinium spp, as well as species grouped in the Areoligera complex and Cordosphaeridium complex (e.g., Pross and Brinkhuis, 2005;Frieling and Sluijs, 2018).The distribution pattern of various chronostratigraphic index taxa such as Biconodinium longissimum and several Wetzelielloids further confirm assignment to ETM-2 and H2.The Senegalinium 235 complex, particularly abundant in the lower part of the section, tolerated low salinities, suggestive of strong seasonal river runoff, in apparent contradiction to the absence of terrestrial pollen and spores (Brinkhuis et al., 2006;Sluijs and Brinkhuis, 2009).
Dinocyst abundance variability shows surprisingly little correspondence to climatic trends.Wetzelielloids, which includes the Apectodinium complex and Apectodinium spp., show (quasi-)global abundance peaks across the PETM but demonstrate a 240 small relative increase prior to ETM2 with their highest relative abundances occurring several meters above both ETM2 and H2 (Crouch et al., 2003;Frieling and Sluijs, 2018; Figure 5).The only anomaly that clearly corresponds to a hyperthermal is the peak abundance of Goniodomideae during H2 (Figure 5).Goniodomideae represent a thermophile group thriving in lagoonal environments (variable salinity) and further offshore under strongly stratified conditions (e.g., Reichart and Brinkhuis, 2003;Frieling and Sluijs, 2018).Taking into account the decreasing abundance of the Senegalinium complex, this likely points 245 to an increase in salinization during this interval, due to decreased runoff or increased evaporation.This interval also yields anomalous abundances of benthic foraminifer linings, which generally indicates a temporary reduction in sedimentation, further supporting the interpretation of decreased runoff during this interval.

Clay Mineralogy
Clay mineralogy shows moderate variation throughout the studied interval.However, these variations are not correlated with 250 other proxies, and the timing of the mineralogic changes is offset relative to the CIEs of ETM2 and H2; i.e. the shift in mineralogy begins prior to the CIE of ETM2 and after the CIE of H2.There is a marked increase in illite during the body of ETM2, and an additional increase in the illite/smectite ratio immediately prior to H2 (Figure 2).There is no detectable kaolinite within the studied interval.

Oxygen Isotope Paleothermometry 255
There is a long term rise in temperature over the studied interval, rising from 11.1°C (-2.8/+2.9)at the base of the core to 13.4°C (-2.9/+3.0)at the top (Figure 2).The largest source of error associated with the δ 18 O temperature measurements is the uncertainty associated with local δ 18 OSW.According to the Fairbanks (1982) equation, a change in salinity of ±1 psu when propagated across calculations can result in an error in temperature on the order of ±2.9°C depending on local freshwater δ 18 O.This error cannot be assumed to be constant throughout the core, as hydroclimate is expected to change dramatically during 260 this time period, which could impact local salinity, as often seems to be the case with the PETM.Agreement in temperature trends between δ 18 O and TEX86 suggests this effect is minimal.Bottom water temperatures during ETM2 were measured to be 11.6°C (+2.9/-2.8),0.9°C warmer than the baseline value.The greatest warming is seen following the CIE associated with H2 at 14.4°C (+3.0/-2.9),2.3°C warmer than the baseline.Peak warming seems decoupled from minimum δ 13 C values for both ETM2 and H2 (Figure 2).These temperature variations are likely the minimum estimates given the resolution of the record.265

TEX86 Biomarker Paleothermometry
GDGT distributions indicate a dominant pelagic Thaumarchaeotal source based on low abundances of terrestrial GDGTs of and lipids derived from methanotrophs or methanogenetic microorganisms (Supplement Figure 3).GDGT2/3 ratios between 1.6 and 2.1 (Appendix 4) indicate dominant lipid origin from the upper ~150 meters of the water column (Hurley et al., 2018;Van der Weijst et al., 2022).Although this may imply contributions from sub-thermocline Thaumarchaeota potentially 270 compromising direct quantitative SST assessment based on the surface sediment calibration dataset, we present the results as SST reconstructions following various linear and non-linear calibrations following convention (Van der Weijst et al., 2022;Hollis et al., 2019).Variability in the TEX86 record should reflect SST variability very well (Ho and Laepple, 2016).
SST based on TEX86 vary by a few degrees, largely mirroring the benthic bottom water temperature trends (Figure 2).
Temperatures appear to increase prior to the body of ETM2 during the pre-onset excursion.Using a linear calibration, SST 275 reached a maximum of ~30°C, ~1.1°C higher than the baseline.Exponential calibrations result in slightly lower SSTs and slightly muted warming (SI).TEX86 reaches minimum values during the CIE associated with ETM2, at ~28°C, 1.0°C colder than baseline values.The trend reverses and reaches a maximum following the CIE associated with H2 at ~31°C, ~1.3°C warmer than the baseline value.

Stratigraphy and Sedimentation
Documented sections spanning the ETM2 in marine shelf settings are rare compared to sections spanning the PETM.The discovery of two smaller hyperthermals, ETM2 and H2, in addition to the PETM, on the mid-Atlantic coast along provides a unique opportunity to assess how regional climate responded to two distinct levels of greenhouse forcing.Apart from biochronostratigraphy, the key to identifying ETM2 is the stable carbon isotope stratigraphy, specifically the presence of the 285 paired CIEs of ETM2 and H2, which at Knapps Narrows is broadly consistent with trends observed elsewhere (Figure 1).The magnitude of the CIE here is intermediate to records of the ETM2 in open marine and continental settings but is consistent with observations of the PETM CIE in marginal marine settings (John et al., 2008;Tipple et al., 2011;Sluijs and Dickens, 2012).The marine carbonates of ETM2 document a smaller excursion than those observed in continental sections and in marine organic carbon (Tipple et al., 2011;Stap et al., 2010;Westerhold et al., 2018;Abels et al., 2015;Sluijs and Dickens, 290 2012).The dampening of the magnitude of change in marine carbonates has been attributed to dissolution and carbonate chemistry changes during the peak of the hyperthermal (e.g.PETM).The enhanced terrestrial signal has been attributed to changes in soil conditions, humidity, plant communities and physiology, and an increase in the organic 13 C fraction (Bowen et al., 2004;Tipple et al., 2011;Sluijs and Dickens, 2012).
Although the interval with low carbonate content prevented the generation of a more detailed foraminiferal isotope record, the 295 covariance between bulk carbonate and benthic foraminiferal δ 13 C throughout combined with the biostratigraphy and magnetostratigraphy confirms our interpretation of the carbon isotope stratigraphy.The onset of the PETM in the mid-Atlantic region is characterized by a clay or low carbonate layer which has been attributed to both coastal acidification and enhanced siliciclastic sediment fluxes that are climatically driven (Bralower et al., 2018;Gibson et al., 2000;Kopp et al., 2009).Highresolution climate simulations are consistent with enhanced seasonal or peak precipitation, which likely increased rates of 300 erosion and sedimentation (Carmichael et al., 2017;Rush et al., 2021).Similarly, there are two potential explanations for the low carbonate interval associated with the body of ETM2: coastal acidification and enhanced siliciclastic flux.Coastal acidification would dissolve carbonate directly, leading to the low carbonate interval.It is unlikely that elevated CO2 levels alone could drive this given the rates of estimated C emissions (e.g., Harper et al., 2020).It is possible that this effect could be enhanced by increased oxidation of organic matter, both in situ and transported into the shelf (Bralower et al., 2018;Lyons et 305 al., 2019).Enhanced siliciclastic transport, is likely the main cause but to a much lesser extent with an estimated 50-100% increase in supply rates during ETM2, as compared to estimates of 2.8 to 220-fold increases with the PETM (Stassen et al., 2012b).Although, recent studies have suggested much more moderate increases in sedimentation rates, which is in good agreement with observations here (Li et al., 2022).
Although the coarse resolution of the carbon isotope stratigraphy might not capture a brief increase in sedimentation rates, our 310 age model indicates sedimentation rates remained fairly constant across ETM2 and H2, in contrast to the PETM.Assuming https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.continuous sedimentation, rates range from 4.6 to 10.8 cm/kyr, with the lower section of the core trending lower and the upper section of the studied interval trending higher with an inflection point at 91.8 m.The lowest rates occurred prior to ETM2, and the highest occurred during the background state between ETM2 and H2 (Figure 6).However, sedimentation rates in shelf settings can vary on timescales shorter than the resolution of our age model is able to detect.It is almost certain that the records 315 reflect amalgamation of periods of enhanced sedimentation and non-deposition (Trampush and Hajek, 2017).Abrupt transitions within the distribution patterns of several dinocyst species suggest minor disconformities, condensed intervals, or depositional hiatuses that would result in highly variable sedimentation rates we cannot record based on the resolution of our carbon isotope age model.Therefore, the age model should be seen as capturing the average over long time periods.
The sedimentation rates predicted based upon the age model are largely consistent with those calculated based on spectral 320 analysis, which found long-term average sedimentation rates on the order of 5 cm/kyr in the lower section of the core and 13 cm/kyr in the upper section, with an inflection point at the same location as predicted by the age model at ~92 m (Figure 4).
Based on spectral characteristics the lower section of the core exhibits cyclicity that could be orbitally paced, while the upper section does not.This is unsurprising given the generally discontinuous or episodic nature of sediment accumulation in such facies.Despite the aforementioned issues surrounding the stochastic nature of shelf deposition, cyclostratigraphic methods 325 have been used in calculating shelf sedimentation rates for the PETM in the region, and the general agreement between the carbon isotope record and the spectral analysis suggests the general interpretation of a bimodal sedimentation pattern is accurate (Li et al., 2022).
Other factors may have a role in observed sedimentation rates, such as the paleobathymetry and local sea level.Faulting within the region could have led to the development of horst and graben complexes (Supplement Figure 1).These positive and 330 negative structural features could produce localized paleobathymetric highs and lows that could have influenced the observed variation in local sedimentation rates (Sabat 1977;Self-Trail et al., 2017a).

Regional Hydrology and Depositional Setting
Evidence supporting a modest intensification of the hydrologic cycle includes the shift in clay mineralogy from smectitedominated facies to illite-dominated facies during the body of ETM2, as well as a moderate increase in illite prior to H2, with 335 no detectable levels of kaolinite.Clay mineralogy is used extensively as a proxy for chemical weathering, with a spectrum from smectite to illite to kaolinite being associated with increasingly wet and warm conditions (Singer, 1980).Many locations experience a dramatic increase in kaolinite during the PETM (Robert and Kennett, 1994;Gibson et al, 2000;Kemp et al., 2016;Chen et al., 2016).Previous studies on clay mineralogy over the PETM in the Salisbury Embayment demonstrated that kaolinite increased, and that this increase could have resulted from the reworking of older deposits (Gibson 2000;John et al., 2012).In 340 other regions, the increase in kaolinite is believed to result from an authigenic weathering response to the PETM (Clechenko et al., 2007;Chen et al., 2016) (Rush et al, unpub. data;Pogge von Strandmann et al., 2021;Ramos et al., 2022).Since illite is a product of weathering between smectite and kaolinite, alongside recent studies 345 demonstrating an immediate weathering response during the PETM, its presence seems support the interpretation that the clays formed in situ during ETM2.As the warming of ETM2 is intermediate to that of the background state and the PETM, the environmental perturbations during ETM2 would likely illicit a weathering response intermediate to the background state and that of the PETM.
The palynological assemblages imply a complicated story.The assemblages are typical of those seen in neritic settings (Pross 350 and Brinkhuis, 2005;Frieling and Sluijs, 2018).They show a large degree of variability in fresh-water run-off, stratification and productivity, but the majority of these changes shows no relation to δ 13 C or temperature change, atypical for dinocysts (Figure 5).
An overall shallowing trend in the upper section of the core may be inferred based on a long-term decline in the abundance of the low-salinity-tolerant Senegalinium complex and a rise of the Goniodomids, which typically occur in massive (harmful) 355 blooms in (seasonally) high salinity, warm, lagoonal conditions in modern and ancient settings (Zonneveld et al., 2013;Frieling and Sluijs, 2018;Sluijs et al., 2018).This shallowing trend also indicates a shift from brackish conditions to more saline conditions upsection that do not necessarily correspond to hyperthermal events and may indicate an overall longer-term trend.
Increased abundances of the low-salinity-tolerant Senegalinium complex and the supply of terrestrial palynomorphs suggest increased river run off in shelf sections across hyperthermal events (e.g., Crouch et al., 2003;Sluijs and Brinkhuis, 2009;Sluijs 360 et al., 2009).These abundances in Knapps Narrows show no correspondence to the CIEs or temperature anomalies.The complex was abundant before, during and after ETM2 but was not present during H2 after its gradual decline, which does not support a strong hydrological response to this event.Terrestrial palynomorphs are rare throughout the record, reminiscent of the Upper Paleocene and Lower Eocene of the New Jersey Shelf (e.g., Zachos et al., 2006).This suggests changes in transport of terrestrial palynomorphs, or that the hinterland was sparsely vegetated in the region, also explaining the lack of response 365 during the PETM, ETM2 and H2 (Self-Trail et al., 2017b).This may be consistent with the paleolatitude of about 33°N (paleolatitude.orgversion 2.1; van Hinsbergen et al., 2015), which arguably would represent an arid zone.
Goniodomids reach a maximum at H2 that also corresponds to the highest reconstructed temperatures.A rise in salinity seems to precede this warming but it should be noted that dinoflagellate blooms are strongly seasonal and therefore should not necessarily correspond to our salinity estimates.The correspondence of Goniodomid abundances with transient global 370 warming events has been previously recorded, for example in brief intervals of the PETM on the New Jersey Shelf, the Nigerian Shelf, and across the Middle Eocene climatic optimum in the eastern equatorial Atlantic ( (Sluijs and Brinkhuis, 2009;Frieling et al., 2017;Cramwinckel et al., 2019).Very high stable carbon isotope ratios suggest these Goniodomid abundances indicate https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.that at least some of these abundances represent an increase in the geographical range and frequency of harmful blooms (Frieling et al., 2017;Sluijs et al., 2018).375 In contrast to the records here, the Lomonosov Ridge palynological assemblages demonstrate a marked shift during ETM2, with an increase in dinoflagellate markers indicative of freshening and eutrophication, and the appearance of palm pollen indicating high winter temperatures (Sluijs et al., 2009;Willard et al., 2019).Further evidence of hydrologic changes is noted in hydrogen isotope records of leaf wax n-alkanes, which demonstrate a large rise during the PETM, a rise prior to ETM2, and a drop during the body of the event as well as during H2 ( (Pagani et al., 2006;Krishnan et al., 2014).380 Looking at other trackers of hydrologic changes across hyperthermals, records of enhanced siliciclastic flux were archived at Mead and Dee Streams in New Zealand across several hyperthermal events (Nicolo et al., 2007;Slotnick et al., 2012).While focused more on biotic and ocean chemistry conditions, the Nile Basin is also interpreted as having a similar response to the PETM but on a smaller scale (Stassen et al., 2012a).Such evidence of enhanced hydrology driving siliciclastic fluxes is also found in hemipelagic sections such as the Terche section of northeastern Italy (D'Onofrio et al., 2016).385 The records from continental sections suggest changes in the regional precipitation patterns were not entirely uniform.An increase in siliciclastic sediment flux in a terrestrial section at Ellesmere Island is interpreted as evidence of an intensified hydrologic system during ETM2 (Reinhardt et al., 2022).The Bighorn Basin record suggests contrasting regional climatic changes between the PETM and later hyperthermal events (Abels et al., 2016), in line with observations from this study.
The overall hydrologic response to ETM2 appears much more modest than that of the PETM, and somewhat muted given the 390 scale of global warming and assuming a roughly linear response of precipitation patterns to global warming.Given that the baseline temperature prior to ETM2 is a few degrees higher than prior to the PETM, it is possible that sensitivity to an equivalent temperature perturbation would be less extreme than for the PETM, i.e. a state dependent response.This also suggests variability in hydroclimate responses to greenhouse forcing in different regions.While the Arctic records demonstrate a strong hydrological response across ETM2, here the interpretation is less straightforward (Sluijs et al., 2009).Depending on 395 the region, local hydroclimate seems to respond in a somewhat non-linear fashion to global warming, characterized more by abrupt mode shifts (e.g., frequency of extremes, or duration of wet vs. dry seasons) as opposed to gradational changes (e.g., mean annual precipitation).In the mid-Atlantic region, our palynological record appears to confirm this overall 'mild' response to the ETM2.In effect, to record only minor changes is rather unique, given that other transient climatic events in the Cenozoic are typically characterized by massive concomitant dinocyst assemblage changes.400

Temperature
There is broad correspondence between temperature variability derived from TEX86 and δ 18 O, suggesting the trends are during the body, and an increase of 1.3°C during H2.The magnitude of these shifts is much lower than those of the PETM at nearby sites, with paleotemperature reconstructions from the PETM derived from TEX86 and δ 18 O at other sections in the 405 region suggesting 5-8 °C surface warming without much scatter (Sluijs et al., 2007;Zachos et al., 2006;Babila et al., 2022).
The elevated temperatures prior to the CIE associated with ETM2 are followed by a drop in temperatures during the event.
There are multiple potential explanations for the apparent lack of correspondence between the CIEs and temperature variability.
In shelf settings, deposition is generally discontinuous and variable, particularly during periods of dramatic climatic changes such as Eocene hyperthermals (Trampush and Hajek, 2017).This has the potential to introduce preservation and sampling 410 biases within the sedimentary record, and given the coarse resolution of the temperature records, the possibility of not capturing a warming anomaly that lasted only a few tens of thousands of years.Another possibility is the contribution of regional scale changes resulting from changing ocean and atmospheric circulation patterns in a modestly warmer world.With the current warming, changes to the Atlantic Meridional Ocean Circulation results in regionally lower temperatures in the North Atlantic (Jackson et al., 2015).Another factor to consider is orbital forcing.The Eocene hyperthermals appear to coincide with 400-415 kyr eccentricity maxima (Lourens et al., 2005;Zachos et al., 2010).Although the lower frequency cyclical changes in CO2 appear to be paced by eccentricity (Zeebe et al., 2017;Vervoort et al., 2021), depending on the phase of precession, local temperature, particularly on or near land, could deviate from global patterns (Kiehl et al., 2018).This effect would be further exaggerated if local sediment deposition tended to be biased toward one phase.The variation in responses between various proxy systems and muted responses relative to the PETM highlights the nonlinearity of the climate system to CO2 forcing and 420 demonstrates the importance of considering additional forcings.The ETM2 has already been documented in numerous pelagic sections, which uniformly demonstrate 2-4°C of warming during the event (Harper et al., 2018).The uniformity observed in open ocean sections is mirrored in previously documented shelf sections.The central Lomonosov Ridge has been argued to have experienced an increase in warm, wet conditions during ETM2, similar to reconstructions for the PETM (Sluijs et al., 2009;Sluijs et al., 2020).TEX86 records for this site indicate a 425 warming on the order of 3-5°C.

Conclusions
This study provides the first documentation of ETM2 and H2 in the Salisbury Embayment of the mid-Atlantic Coastal Plain. the short-lived hyperthermals and point more towards being dominated by long-term trends.Similarly, sedimentation rates 435 also appear to be dominated by long term trends, occurring in a bimodal fashion with the transition between the states having no correlation to either of the hyperthermals.When compared to the PETM and other records of ETM2, these contrasting records demonstrate the complexities of climatic changes and regional variability associated with rapid warming events, highlighting the need for extensive spatial coverage to interpret climatic changes.In essence, the local ∆precipitation did not scale linearly with the ∆temperature.The relatively muted signal in this region compared to the PETM may be due in part to 440 the warmer baseline state prior to ETM2 compared to the PETM.Future research into high-resolution climate models of these smaller scale hyperthermal events may assist in interpretation of this record as well as records in different regions.

Data Availability
All data is available via the Pangaea repository (doi pending)

Sample Availability 445
Samples are available upon request from the Florence Bascom Geoscience Center of the United States Geological Survey.

Author Contributions
Manuscript was prepared by WR, with contributions from all co-authors.Stable isotope measurements, clay mineralogy, carbonate content, age model, and data synthesis were performed by WR.Coring and sampling was performed by JS and MR.
Paleomagnetic work and spectral analysis were performed by YZ.Nannofossil biostratigraphy was performed by JS.TEX86 450 measurements were performed by AS.Palynology was performed by HB.Project oversight and mentoring was provided by JZ.

Competing interests
The authors declare that they have no conflict of interest.level and relatively more (# 6-7) of contributing astronomical parameters are identified, i.e., at ~1 cm/kyr, ~5 cm/kyr, and ~13 cm/kyr.For the eCOCO analysis, the sliding window is 18 m using 0.2-m step.The target astronomical solution is La2004 at 53 Ma (with 7 astronomical cycles at 405, 125, 95, 39.2, 23.1, 21.9, and 18.8 kyr, respectively).All periodograms were analyzed with the AR(1) red noise model removed.The number of Monte Carlo simulations is 2000.The tested sedimentation rates range from 0 to 15 cm/kyr with a step of 0.1 cm/kyr.Note that from depth of ~82 m -90 m that covers the studied interval, a change of sedimentation rates 805 exists from ~5 cm/kyr to ~13 cm/kyr.The evolutionary power spectrum (Supplementary figure 5) of this interval also indicates a distortion of cyclicity.Foraminifera linings can be used to infer periods of rapid or low siliciclastic influx.Wetzelielloids are associated with periods of high productivity, Goniodomids are associated with high temperatures and strong salinity stratification, and the Senegalinium complex is associated with low salinity (Frieling and Sluijs, 2018).Of note is a long-term increase in salinity from relatively fresh conditions at the base of the core to more saline conditions upsection.Rapid changes in abundance of Wetzelielloids suggests disconformities, . Work on the radioisotopes of the clay minerals, namely strontium and lead, demonstrate little https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.change in the sourcing of sediment in the Atlantic coastal plain during the PETM and work on lithium isotopes seems to suggest rapid changes to global weathering patterns during the PETM accurate.TEX86 records indicate minor variations with a rise of 1°C prior to ETM2, a drop of 1°C relative to the baseline https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.
Clay mineralogy, δ 18 O, and TEX86 records portray climate perturbations across this interval.The shift in clay mineralogy from smectite to illite taken in the context of kaolinite dominated facies during the preceding PETM suggests the clay mineralogy 430 changes are authigenic and represent an intermediate stage of weathering relative to the PETM and the background state resulting from an enhanced hydrologic cycle.Temperature proxies show minor increases in temperature that appear asynchronous with changes to the carbon cycle and may represent alternative forcings such as changes in ocean and atmospheric circulation or orbital forcing or a potential sampling artifact.Dinocyst assemblages demonstrate little response to https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.
trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.11. Acknowledgements Funding for this project was provided by NSF OCE-1415958, OCE-1658017, and OCE-2103513.We thank Antoinette van den Dikkenberg, Giovanni Dammers and Natasja Welters (Utrecht University) for analytical and technical assistance.AS 460 thanks the European Research Council for Consolidator Grant #771497 (SPANC).Core collection and MMR were funded by the USGS Climate Research and Development Program.JST was funded by the USGS National Cooperative Geologic Mapping Program.YZ and paleomagnetic work was funded by the Geologic Timescale Foundation.https://doi.org/10.5194/egusphere-2023-58Preprint.Discussion started: 27 January 2023 c Author(s) 2023.CC BY 4.0 License.

Figure 6 :
Figure 6: Sedimentation rates in the Knapps Narrows core based on correlations of δ 13 C inflection points to the age model of Lauretano et al., 2015 at Walvis Ridge Site 391.Sedimentation rates in cm/kyr.Note relative consistency compared to orders of magnitude changes that were documented during the PETM in Stassen et al. (2012b), with moderate increases beginning at 91.8 m, 820