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The late Paleocene and early Eocene (LPEE; ca. 59 to 52 Ma) was an interval of warming superposed on the abrupt carbon release events (or hyperthermals) of the early Eocene. The LPEE provides an opportunity to test the coupling of carbon release (and associated long- and short-term warming), to other climate-carbon cycle feedbacks (e.g., ocean acidification and shifts in the hydrologic cycle). Much work has focused on determining the sensitivities of sea surface pH, climate and the hydrologic cycle to carbon release during the largest hyperthermal of the Cenozoic, the Paleocene-Eocene Thermal Maximum (PETM; ca. 56 Ma). However, Eocene Thermal Maximum 2 (ETM-2; ca. 54 Ma) provides a secondary calibration point on assessing the sensitivity of carbon cycle-climate feedbacks to carbon release. Additionally, it has been hypothesized that the long-term carbon storage and release mechanisms (e.g., marine clathrates and organic carbon) are linked to the hyperthermal events, however, constraints on long-term changes in carbonate chemistry are lacking for the interval.This thesis aims to characterize the nature of changes in the climate and carbon cycle associated with the onset of the LPEE warming, and subsequent cyclicity up to ETM-2 with the overarching goal of testing hypotheses for the origin of both the LPEE and ETM-2. To this end, a multiproxy approach utilizing boron- based (δ11B and B/Ca) and paleothermometry proxies (Mg/Ca and δ18O) is applied to determine the magnitude of shifts in climate, marine carbonate chemistry, and, on shorter timescales, the hydrologic cycle using materials collected from ODP Sites 1209, 1210, 1262 and 1265 (Legs 198 and 208; subtropical Pacific and Atlantic, respectively). Further, I present the first LOSCAR carbon cycle/climate numerical simulations to model the release of carbon to the atmosphere during ETM-2, constrained by the CCD, δ13C, and proxy-based observations of changes in temperature and pH. Additionally, changes in long-term Pacific sea surface pH are used to assess the validity of previously published carbon cycle simulations of the LPEE.

The Early Cretaceous is characterized by warm background temperatures (i.e., greenhouse climate) and carbon cycle perturbations that are often marked by ocean anoxic events (OAEs) and associated shifts in the hydrologic cycle. Higher-resolution records of terrestrial and marine δ13C and δ18O (both carbonates and organics) suggest climate shifts during the Aptian–Albian, including a warm period associated with OAE 1a in the early Aptian and a subsequent “cold snap” near the Aptian–Albian boundary prior to the Kilian and OAE 1b. Understanding the continental system is an important factor in determining the triggers and feedbacks to these events. Here, we present new paleosol carbonate stable isotopic (δ13C, δ18O and Δ47) and CALMAG weathering parameter results from the Xiagou and Zhonggou formations (part of the Xinminpu Group in the Yujingzi Basin of NW China) spanning the Aptian–Albian. Published mean annual air temperature (MAAT) records of the Barremian–Albian from Asia are relatively cool with respect to the Early Cretaceous. However, these records are largely based on coupled δ18O measurements of dinosaur apatite phosphate (δ18Op) and carbonate (δ18Ocarb) and therefore rely on estimates of meteoric water δ18O (δ18Omw) from δ18Op. Significant shifts in the hydrologic cycle likely influenced δ18Omw in the region, complicating these MAAT estimates. Thus, temperature records independent of δ18Omw (e.g., clumped isotopes or Δ47) are desirable and required to confirm temperatures estimated with δ18Op and δ18Oc and to reliably determine regional shifts in δ18Omw. Primary carbonate material was identified using traditional petrography, cathodoluminescence inspection, and δ13C and δ18O subsampling. Our preliminary Δ47-based temperature reconstructions (record mean of 14.9 ∘C), which we interpret as likely being representative of MAAT, match prior estimates from similar paleolatitudes of Asian MAAT (average ∼ 15 ∘C) across the Aptian–Albian. This, supported by our estimated mean atmospheric paleo-pCO2 concentration of 396 ppmv, indicates relatively cooler midlatitude terrestrial climate. Additionally, our coupled δ18O and Δ47 records suggest shifts in the regional hydrologic cycle (i.e., ΔMAP, mean annual precipitation, and Δδ18Omw) that may track Aptian–Albian climate perturbations (i.e., a drying of Asian continental climate associated with the cool interval).

Several studies indicate that North Atlantic Deep Water (NADW) formation might have initiated during the globally warm Eocene (56–34 Ma). However, constraints on Eocene surface ocean conditions in source regions presently conducive to deep water formation are sparse. Here we test whether ocean conditions of the middle Eocene Labrador Sea might have allowed for deep water formation by applying (organic) geochemical and palynological techniques, on sediments from Ocean Drilling Program (ODP) Site 647. We reconstruct a long‐term sea surface temperature (SST) drop from ~30°C to ~27°C between 41.5 to 38.5 Ma, based on TEX86. Superimposed on this trend, we record ~2°C warming in SST associated with the Middle Eocene Climatic Optimum (MECO; ~40 Ma), which is the northernmost MECO record as yet, and another, likely regional, warming phase at ~41.1 Ma, associated with low‐latitude planktic foraminifera and dinoflagellate cyst incursions. Dinoflagellate cyst assemblages together with planktonic foraminiferal stable oxygen isotope ratios overall indicate low surface water salinities and strong stratification. Benthic foraminifer stable carbon and oxygen isotope ratios differ from global deep ocean values by 1–2‰ and 2–4‰, respectively, indicating geographic basin isolation. Our multiproxy reconstructions depict a consistent picture of relatively warm and fresh but also highly variable surface ocean conditions in the middle Eocene Labrador Sea. These conditions were unlikely conducive to deep water formation. This implies either NADW did not yet form during the middle Eocene or it formed in a different source region and subsequently bypassed the southern Labrador Sea., Key Points The middle Eocene Labrador Sea was characterised by warm, low‐salinity surface watersWe find two phases of superimposed sea surface warming and subtropical plankton incursions, including the Middle Eocene Climatic OptimumBottom water stable carbon and oxygen isotope ratios indicate isolated basin

This thesis examined the reliability and potential paleoceanographic applications of multiple chemical and isotopic proxies in a variety of benthic foraminifera species. The selected proxies are known to resolve organic matter flux and bottom water oxygenation, simultaneously allowing for examination of microhabitat effects in species with diverse depths of calcification. Globally distributed core-top samples from 19 different sites yielded a total of 17 varieties of benthic foraminifera. Samples were analyzed for [delta]¹³C and [delta]¹⁸O values to determine species depth distributions and interspecies vital differences in [delta]¹⁸O. A Mg-cleaning method was employed before samples were analyzed for the elemental ratios; U/Ca, Mn/Ca, B/Ca, and Mg/Ca to elucidate species elemental incorporation differences in various locations. Modeled [delta]¹³C pore water curves were applied in two different sites to determine calcification depth distribution of benthic foraminifera. [delta]¹³C offset differences between C. wuellerstorfi and Oridorsalis spp., were determined at every possible site and evaluated as a potential carbonate chemistry proxy, showing notable success. Positive correlations were observed between [delta]¹³C offset, and pH and carbonate ion concentration, with R2 values of 0.67 and 0.67, respectively. Stronger correlations between [delta]¹³C offset differences and temperature were observed (R² of 0.73), suggesting the proxy is being influenced by physical effects, not solely chemical. Epifaunal C. wuellerstorfi showed higher variability in U/Ca test coatings and in Mn/Ca test incorporation when compared to the [delta]¹³C differences between epifaunal and infaunal species, while infaunal Oridorsalis spp. showed tighter interdependence suggesting microhabitat depth plays a significant role in the variability of elemental incorporation

International Ocean Discovery Program (IODP) Expedition 371 drilled six sites in the Tasman Sea of the southwest Pacific between 27 July and 26 September 2017. The primary goal was to understand Tonga-Kermadec subduction initiation through recovery of Paleogene sediment records. Secondary goals involved understanding regional oceanography and climate since the Paleogene. Six sites were drilled, recovering 2506 m of cored sediment and volcanic rock in 36.4 days of on-site drilling during a total expedition length of 58 days. Wireline logs were collected at two sites. Shipboard observations made using cores and logs represent a substantial gain in fundamental knowledge about northern Zealandia, because only Deep Sea Drilling Project Sites 206, 207, and 208 had penetrated beneath upper Eocene strata within the region. The cored intervals at five sites (U1506-U1510) sampled nannofossil and foraminiferal ooze or chalk that contained volcanic or volcaniclastic intervals with variable clay content. Paleocene and Cretaceous sections range from more clay rich to predominantly claystone. At the final site (U1511), a sequence of abyssal clay and diatomite was recovered with only minor amounts of carbonate. The ages of strata at the base of each site were middle Eocene to Late Cretaceous, and our new results provide the first firm basis for defining formal lithostratigraphic units that can be mapped across a substantial part of northern Zealandia and related to onshore regions of New Caledonia and New Zealand. The material and data recovered during Expedition 371 enable primary scientific goals to be accomplished. All six sites provided new stratigraphic and paleogeographic information that can be put into context through regional seismic-stratigraphic interpretation and hence provide strong constraints on geodynamic models of subduction zone initiation. Our new observations can be directly related to the timing of plate deformation, the magnitude and timing of vertical motions, and the timing and type of volcanism. Secondary paleoclimate objectives were not all completed as planned, but significant new records of southwest Pacific climate were obtained.

International Ocean Discovery Program (IODP) Expedition 371 drilled six sites in the Tasman Sea of the southwest Pacific between 27 July and 26 September 2017. The primary goal was to understand Tonga-Kermadec subduction initiation through recovery of Paleogene sediment records. Secondary goals involved understanding regional oceanography and climate since the Paleogene. Six sites were drilled, recovering 2506 m of cored sediment and volcanic rock in 36.4 days of on-site drilling during a total expedition length of 58 days. Wireline logs were collected at two sites. Shipboard observations made using cores and logs represent a substantial gain in fundamental knowledge about northern Zealandia, because only Deep Sea Drilling Project Sites 206, 207, and 208 had penetrated beneath upper Eocene strata within the region. The cored intervals at five sites (U1506-U1510) sampled nannofossil and foraminiferal ooze or chalk that contained volcanic or volcaniclastic intervals with variable clay content. Paleocene and Cretaceous sections range from more clay rich to predominantly claystone. At the final site (U1511), a sequence of abyssal clay and diatomite was recovered with only minor amounts of carbonate. The ages of strata at the base of each site were middle Eocene to Late Cretaceous, and our new results provide the first firm basis for defining formal lithostratigraphic units that can be mapped across a substantial part of northern Zealandia and related to onshore regions of New Caledonia and New Zealand. The material and data recovered during Expedition 371 enable primary scientific goals to be accomplished. All six sites provided new stratigraphic and paleogeographic information that can be put into context through regional seismic-stratigraphic interpretation and hence provide strong constraints on geodynamic models of subduction zone initiation. Our new observations can be directly related to the timing of plate deformation, the magnitude and timing of vertical motions, and the timing and type of volcanism. Secondary paleoclimate objectives were not all completed as planned, but significant new records of southwest Pacific climate were obtained.

Eocene Thermal Maximum 2 (ETM-2; ca. 54.2 Ma) represents the second largest of the major Eocene hyperthermals, yet comparatively little is known about the scale and rate of climatic change for key regions. Here we provide the first detailed records of subtropical sea-surface warming and salinization for ETM-2 at two subtropical locations, Ocean Drilling Program Sites 1209 (North Pacific) and 1265 (South Atlantic). Coupled planktic foraminiferal Mg/Ca and δ18O indicate 2-4 °C of rapid warming and local salinization of ~1-2 ppt at both sites. The increase in sea-surface temperature is equivalent to anomalies reported from higher latitude sites, and is consistent with theory on the expected pattern of spatial temperature response to greenhouse gas forcing in an ice-free world (i.e., no ice-albedo feedback). Similarly, the observed salinization is consistent with the hypothesis of enhanced meridional vapor transport and increased subtropical aridity in a warmer world.