2.1 Material

The Knapps Narrows core was drilled at 38.72129^∘ N, 76.33162^∘ W 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 (Fig. 1). The cored target interval lies between 84–102 m in the Nanjemoy Formation, an Eocene glauconitic sand in the upper Pamunkey group deposited in a shallow marine environment (Darton, 1951; Self-Trail et al., 2017b). From the bottom of the section to 88.4 m, the 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 m 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 are found upsection, and the clay content decreases. The sediments are heavily bioturbated throughout. The Nanjemoy typically overlays Marlboro Clay, which was deposited during the early Eocene PETM, with a significant regional unconformity (Darton, 1951; Stassen et al., 2012a) In this location, due to local faulting, the Marlboro Clay is entirely absent, and an expanded, slightly younger, lower Eocene interval equivalent to the calcareous nannofossil biozone NP11, which contains ETM2, is present (Martini, 1971; 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 Fig. A1). The initial core description noted much coarser grain sizes when compared to the lithological expression of the Nanjemoy in nearby cores, further supporting the interpretation of rapid infilling during this time. All raw data are included in the supplemental datasets that are available through PANGAEA (see the “Data availability” section at the end of the paper).

Figure 1Map showing the location of Knapps Narrows (KN), the South Dover Bridge (SDB), and Howards Tract (HT) coreholes. The Fall Line delimits the crystalline basement rock (gray) from Cretaceous–Cenozoic-age coastal plain sediments (white). Modified from Self-Trail et al. (2017b).

2.2 Methods

3.1 Stratigraphic framework

A distinct magnetic reversal pattern is preserved in the Knapps Narrows core, with the section below 99.21 m (sample KN37) 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 (Fig. 2). The lowermost section is placed in zone NP10, based on the first occurrence (FO) of Rhomboaster spp. and the absence of Tribrachiatus contortus, which has its FO at 96.6 m. The top of zone NP10 is placed at 95.2 m on the last occurrence of T. contortus, and the base of zone NP11 is identified at 95.1 m, based on the FO of T. orthostylus. This reconstructed bio-magnetostratigraphy (Figs. A2 and A3) is generally consistent with the Paleogene timescale 2020 (Speijer et al., 2020) and the timing of ETM2 (Cramer et al., 2003; Lourens et al., 2005; Westerhold et al., 2007; Stassen et al., 2012a).

Figure 2Knapps Narrows core site (from left to right) depth, nannofossil zones, magnetochrons, lithology, carbon isotopes, percent carbonate, clay mineralogy, and benthic δ¹⁸O temperatures, as well as the TEX₈₆ sea surface temperatures over the ETM2 and H2 interval at Knapps Narrows. TEX₈₆ temperatures use linear calibration from O'Brien et al. (2017). The error in δ¹⁸O represents 1 PSU change in salinity. Note the logarithmic scale for percent carbonate and low-carbonate interval at ETM2, spike in the illite–smectite ratio at ETM2, agreement between benthic and sea surface temperatures, and offsets in timing between carbon isotopes, clay mineralogy, and temperature proxies.

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The bulk carbonate δ¹³C record captures a −1.5 ‰ excursion from 98.18–96.41 m before recovering to the baseline, followed by a second −2.5 ‰ excursion between 95.37–93.46 m that peaks at 94.48 m (Fig. 3). This double-peak feature facilitates the recognition of ETM2, as it is documented for several marine and continental sections (e.g., Walvis Ridge and Bighorn Basin in Fig. 3; Cramer et al., 2003; Stap et al., 2010; Abels et al., 2016). Between 93.46 and 88.98 m, the carbon isotope ratios return to near-baseline values before another −1.5 ‰ excursion at 88.51 m, consistent with observations of H2 at other sites (Fig. 3; Cramer et al., 2003; Stap et al., 2010; Abels et al., 2016). Benthic foraminiferal carbon isotope trends largely track those in bulk carbonate. An interval with low-carbonate content (∼ 93–94.5 m; Fig. 2) coincides with the CIE minima at 94.48 m, where the sediments were barren of foraminifera.

Figure 3Carbonate δ¹³C records of ETM2 and H2 at Walvis Ridge (bulk), Bighorn Basin (pedogenic nodules), Shatsky Rise (benthic), and Knapps Narrows (this study). For Knapps Narrows, bulk carbonate values are in gray, and benthic values are in black. Knapps Narrows at 94.5 m was nearly barren of foraminifera. Walvis Ridge data are from Stap et al. (2010), the age model is from Lauretano et al. (2015), Bighorn Basin data are from Abels et al. (2016), and Shatsky Rise data are from Westerhold et al. (2018).

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Figure 4COCO (a–c) and eCOCO (d–f)analysis of the wireline gamma ray log (42.5–103.5 m). The studied interval lies between 84 and 102 m. In the COCO plots, three clusters of high-ρ sedimentation rates, accompanied with a low-H₀ significance level, and relatively more (6–7 parameters) contributing astronomical parameters are identified, i.e., at ∼ 1, ∼ 5, and ∼ 13 cm kyr⁻¹. For the eCOCO analysis, the sliding window is 18 m, using 0.2 m steps. The target astronomical solution is La2004 at 53 Ma (with seven astronomical cycles at 405, 125, 95, 39.2, 23.1, 21.9, and 18.8 kyr, respectively). All periodograms were analyzed, with the autoregressive (AR1) red noise model removed. The number of Monte Carlo simulations is 2000. The tested sedimentation rates range from 0 to 15 cm kyr⁻¹, with a time step of 0.1 cm kyr⁻¹. Note that from depths of ∼ 82–90 m, which cover the studied interval, a change in the sedimentation rate exists from ∼ 5 to ∼ 13 cm kyr⁻¹. The evolutionary power spectrum (Fig. A5) of this interval also indicates a distortion of cyclicity.

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The COCO and eCOCO (Fig. 4) analysis shows a sedimentation rate of ∼ 5 cm kyr⁻¹ for most of the succession from 103.5 to 43.5 m, except for the interval of 90 to ∼ 82 m, where there is a transition to a sedimentation rate of approximately 13 cm kyr⁻¹. Assuming orbital forcing, the ∼ 5 cm kyr⁻¹ and a cycle of ∼ 18 m (Fig. A5) correlates to the 405 kyr long eccentricity period. Given the surge in estimated sedimentation rate, the observed ∼ 18 m cycle should instead correlate to the short eccentricity periodicity. Given the sudden increase in deposition rate, this interval may reflect on other factors; for example, there could be 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.

3.2 Palynology

Palynological associations are dominated by marine palynomorphs, notably dinocysts, and are common to abundant organic linings 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 the abundant contributions of Spiniferites spp., Operculodinium spp., and species grouped in the Areoligera and Cordosphaeridium complexes (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 (e.g., W. articulata and W. tabulatum) further confirm the assignment to ETM2 and H2 (e.g., Williams et al.,2004; Bijl, 2022). The Senegalinium complex, particularly abundant in the lower part of the section, tolerated low salinities, which is suggestive of strong seasonal river runoff and is in apparent contradiction to the absence of terrestrial pollen and spores (Brinkhuis et al., 2006; Sluijs and Brinkhuis, 2009).

3.3 Clay mineralogy

Clay mineralogy shows moderate variation throughout the studied interval. The clay size fraction is dominated by smectite (> 50 % and up to 95 %) over much of the studied interval, with the remainder being composed largely of illite. There is a marked increase in illite during the body of ETM2, which peaks at a 1.5 illite $/$ smectite ratio, and an additional, smaller, increase in the illite $/$ smectite ratio immediately prior to H2 (Fig. 2). There is no detectable kaolinite within the studied interval. These variations are not correlated with 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.

3.4 Oxygen isotope paleothermometry

There is a long-term rise in temperature over the studied interval from 11.1 ^∘C ($-\mathrm{2.8}/+\mathrm{2.9}$) at the base of the core to 13.4 ^∘C ($-\mathrm{2.9}/+\mathrm{3.0}$) at the top (Fig. 2). The largest source of error associated with the δ¹⁸O temperature measurements is the uncertainty associated with local δ¹⁸O_SW. According to the Fairbanks (1982) equation, a change in salinity of ±1 PSU, when propagated across calculations, can result in an error in the temperature on the order of ±2.9 ^∘C, depending on the local freshwater δ¹⁸O. This error cannot be assumed to be constant throughout the core, as hydroclimate is expected to change dramatically during this time period, which could impact local salinity and often seems to be the case with the PETM. Agreement in temperature trends between δ¹⁸O and TEX₈₆ suggests that this effect is minimal. Bottom water temperatures during ETM2 were measured to be 11.6 ^∘C ($+\mathrm{2.9}/-\mathrm{2.8}$), which is 0.9 ^∘C warmer than the baseline value. The greatest warming is seen following the CIE associated with H2 at 14.4 ^∘C ($+\mathrm{3.0}/-\mathrm{2.9}$), which is 2.3 ^∘C warmer than the baseline. Peak warming seems decoupled from minimum δ¹³C values for both ETM2 and H2 (Fig. 2). These temperature variations are likely the minimum estimates given the resolution of the record.

3.5 TEX₈₆ biomarker paleothermometry

SST values based on TEX₈₆ vary by a few degrees, largely mirroring the benthic bottom water temperature trends (Fig. 2). TEX₈₆ records for this site indicate a warming on the order of 3–5 ^∘C over the studied interval. Temperatures appear to increase prior to the body of ETM2 during the pre-onset excursion. Using a linear calibration, SST reached a maximum of ∼ 30 ^∘C, which is ∼ 1.1 ^∘C higher than the baseline. Exponential calibrations result in slightly lower SSTs and slightly muted warming (see the “Data availability” for more information). TEX₈₆ reaches minimum values during the CIE associated with ETM2 at ∼ 28 ^∘C, which 1.0 ^∘C colder than baseline values. The trend reverses and reaches a maximum, following the CIE associated with H2 at ∼ 31^∘C, which is ∼ 1.3 ^∘C warmer than the baseline value.

4.1 Stratigraphy and sedimentation

Documented sections spanning the ETM2 in marine shelf settings are rare compared to those spanning the PETM. The discovery of the lithologic expression of two smaller hyperthermals, ETM2 and H2, in addition to the PETM, on the mid-Atlantic coast provides a unique opportunity to assess how regional climate responded to two distinct levels of greenhouse forcing. Apart from bio-magnetostratigraphy, the key to identifying ETM2 is the stable carbon isotope stratigraphy, specifically the presence of the paired CIEs of ETM2 and H2, which, at Knapps Narrows, is broadly consistent with trends observed elsewhere (Fig. 1). The magnitude of the CIE here is intermediate compared 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., 2016; Sluijs and Dickens, 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 ¹³C fraction (Bowen et al., 2004; Tipple et al., 2011; Sluijs and Dickens, 2012). The relative lack of dissolution in shallower marine environments and lack of controls that enhance the terrestrial signal likely contribute to this more intermediate CIE.

Although the interval with low-carbonate content prevented the generation of a more detailed foraminiferal isotope record, the covariance between bulk carbonate and benthic foraminiferal δ¹³C 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). High-resolution climate simulations are consistent with enhanced seasonal or peak precipitation, which likely increased the rates of 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, namely coastal acidification and enhanced siliciclastic flux. Coastal acidification would inhibit the calcification and potentially dissolve carbonate directly, leading to the low-carbonate interval. In arguing for increased acidification, Bralower et al. (2018) put forth two possibilities, namely a possible shoaling of the lysocline to the middle shelf and a regional shoaling of the lysocline driven by enhanced eutrophication and microbial activity. However, the increase in pCO₂ levels during ETM2 was much lower than that of the PETM (e.g., Harper et al., 2020), and there is little evidence for a global shoaling of the lysocline to this degree during ETM2. Therefore, it is unlikely that elevated CO₂ levels alone could drive dissolution on the shelf in this region, given the rates of estimated C emissions. It is possible that dissolution could be enhanced by increased oxidation of organic matter for both in situ organic matter and organic matter transported into the shelf (Bralower et al., 2018; Lyons et al., 2019). However, the dinoflagellates in our study interval do not suggest significant changes in productivity in response to the CIEs. Enhanced siliciclastic transport may have contributed to the low-carbonate interval as well, even though this increase appears to be to a much lesser extent relative to the PETM with an estimated 50 %–100 % increase in supply rates within the studied interval, as compared to estimates of 2.8 to 220-fold increases within the PETM. Again, the inflection occurs at ∼ 92 m whereupon the sedimentation rate increase is not correlated to either hyperthermal (Stassen et al., 2012b), although recent studies have suggested much more moderate increases in sedimentation rates across the PETM, which is in good agreement with observations here (Li et al., 2022). Lacking a clear indication of a single driver for the low-carbonate interval, it is likely that a confluence of factors drove the observed changes.

Figure 5Knapps Narrows core site (from left to right) depth, nannofossil zones, magnetochrons, lithology, carbon isotopes, foraminifera lining counts, relative abundance of wetzelielloids, relative abundance of Goniodomids, and relative abundance of the Senegalinium complex. 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 the abundance of wetzelielloids suggest disconformities, periods of non-deposition, or condensed sections.

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Figure 6Sedimentation rates in the Knapps Narrows core based on the correlations of δ¹³C inflection points to the age model of Lauretano et al. (2015) at Walvis Ridge site 391. Sedimentation rates are given in centimeters per thousand years. Note the relative consistency compared to the order of magnitude changes that were documented during the PETM in Stassen et al. (2012b), with moderate increases beginning at 91.8 m, which is consistent with cyclostratigraphy.

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Although the coarse resolution of the carbon isotope stratigraphy might not capture a brief increase in sedimentation rates, our age model indicates that the sedimentation rates remained fairly constant across ETM2 and H2, which is in contrast to the lithologic expression of the PETM in this area. Assuming continuous sedimentation, rates range from 4.6 to 10.8 cm kyr⁻¹, 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 (Fig. 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 reflect the 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 that we cannot record, based on the resolution of our carbon isotope age model. Therefore, the age model should be seen to be 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 analysis, which found long-term average sedimentation rates on the order of 5 cm kyr⁻¹ in the lower section of the core and 13 cm kyr⁻¹ in the upper section, with an inflection point at the same location as predicted by the age model at ∼ 92 m (Fig. 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 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 (Fig. A1). These positive and 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).

4.2 Regional hydrology and depositional setting

Evidence supporting a modest intensification of the hydrologic cycle includes the shift in clay mineralogy from smectite-dominated facies to illite-dominated facies during the body of ETM2 and a moderate increase in illite prior to H2, with no detectable levels of kaolinite. Clay mineralogy is used extensively as a proxy for chemical weathering, with a spectrum from smectite and 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 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). Work on the radioisotopes of the clay minerals, namely strontium and lead, demonstrate little 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 (Rush et al., unpublished 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 demonstrating an immediate weathering response during the PETM, its presence seems to 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 and Brinkhuis, 2005; Frieling and Sluijs, 2018). They show a large degree of variability in freshwater runoff, stratification, and productivity, but the majority of these changes show no relation to δ¹³C or temperature change, which is atypical for dinocysts (Fig. 5). Wetzelielloids, which include the Apectodinium complex and Apectodinium spp., show (quasi-)global abundance peaks across the PETM but demonstrate a 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; Fig. 5). The only anomaly that clearly corresponds to a hyperthermal is the peak abundance of Goniodomideae during H2 (Fig. 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 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 indicate a temporary reduction in sedimentation, further supporting the interpretation of decreased runoff during this interval.

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) 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 runoff in shelf sections across hyperthermal events (e.g., Crouch et al., 2003; Sluijs and Brinkhuis, 2009; Sluijs 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 during the PETM, ETM2, and H2 (Self-Trail et al., 2017b). This may be consistent with the paleolatitude of about 33^∘ N (http://www.paleolatitude.org/, version 2.1, last access: 10 August 2023; van Hinsbergen et al., 2015), which arguably would represent an arid zone. Quantitative reconstructions of salinity could aid in this interpretation. However, at present, we are unable to reconstruct surface salinities due to a paucity of surface-dwelling foraminifera species within the studied interval and the qualitative nature of dinoflagellate-based paleoenvironmental reconstructions.

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 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 that these Goniodomid abundances indicate 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).

In contrast to the records here, the Lomonosov Ridge palynological assemblages demonstrate a marked shift during ETM2, with an increase in dinoflagellate markers that are indicative of freshening and eutrophication and the appearance of palm pollen that indicates high winter temperatures (Sluijs et al., 2009; Willard et al., 2019). Further evidence of hydrologic changes is noted in the 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).

Looking at other trackers of hydrologic changes across hyperthermals, records of enhanced siliciclastic flux were archived at Mead and Dee streams in Aotearoa / 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).

The records from continental sections suggest that 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), which is 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 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 the 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 the region, the 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; Carmichael et al., 2017; Rush et al., 2021). 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 and more significant changes in sedimentation.

4.3 Temperature

GDGT distributions indicate a dominant pelagic thaumarchaeotal source based on low abundances of terrestrial GDGTs and of lipids derived from methanotrophs or methanogenetic microorganisms (Fig. A3). GDGT2 $/$ 3 ratios between 1.6 and 2.1 (Fig. A4) indicate a dominant lipid origin from the upper ∼ 150 m of the water column (Hurley et al., 2018; van der Weijst et al., 2022). Although this may imply contributions from sub-thermocline Thaumarchaeota, potentially 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).

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, 2020).

There is broad correspondence between temperature variability derived from TEX₈₆ and δ¹⁸O, suggesting the trends are accurate. TEX₈₆ records indicate minor variations, with a rise of 1 ^∘C prior to ETM2, a drop of 1 ^∘C relative to the baseline 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 δ¹⁸O at other sections in the 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. The first is that we were unable to measure δ¹⁸O during the peak of ETM2 due to the low-carbonate interval, and we may be missing some degree of internal variability within the event. However, the TEX₈₆ record had the lowest temperature recorded within this interval (Fig. 2). 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 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. Another factor to consider is orbital forcing. The Eocene hyperthermals appear to coincide with 400 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, the 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 non-linearity of the climate system to CO₂ forcing and demonstrates the importance of considering additional forcings.