A high‐resolution benthic stable‐isotope record for the South Atlantic: implications for orbital scale changes in Late Paleocene–Early Eocene climate and circulation

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.The Late Paleocene and Early Eocene were characterized by warm greenhouse climates, punctuated by a series of rapid warming and ocean acidification events known as “hyperthermals”, thought to have been paced or triggered by orbital cycles. While these hyperthermals, such as the Paleocene Eocene Thermal Maximum (PETM), have been studied in great detail, the background low-amplitude cycles seen in carbon and oxygen-isotope records throughout the Paleocene–Eocene have hitherto not been resolved. Here we present a 7.7 million year (myr) long, high-resolution, orbitally-tuned, benthic foraminiferal stable-isotope record spanning the late Paleocene and early Eocene interval (∼52.5–60.5 Ma) from Ocean Drilling Program (ODP) Site 1262, South Atlantic. This high resolution (∼2–4 kyr) record allows the changing character and phasing of orbitally-modulated cycles to be studied in unprecedented detail as it reflects the long-term trend in carbon cycle and climate over this interval. The main pacemaker in the benthic oxygen-isotope (δ18O) and carbon-isotope (δ13C) records from ODP Site 1262, are the long (405 kyr) and short (100 kyr) eccentricity cycles, and precession (21 kyr). Obliquity (41 kyr) is almost absent throughout the section except for a few brief intervals where it has a relatively weak influence. During the course of the Early Paleogene record, and particularly in the latest Paleocene, eccentricity-paced negative carbon-isotope excursions (δ13C, CIEs) and coeval negative oxygen-isotope (δ18O) excursions correspond to low carbonate (CaCO3) and coarse fraction (%CF) values due to increased carbonate dissolution, suggesting shoaling of the lysocline and accompanied changes in the global exogenic carbon cycle. These negative CIEs and δ18O events coincide with maxima in eccentricity, with changes in δ18O leading changes in δ13C by ∼6 (±5) kyr in the 405-kyr band and by ∼3 (±1) kyr in the higher frequency 100-kyr band on average. However, these phase lags are not constant, with the lag in the 405-kyr band extending from ∼4 (±5) kyr to ∼21 (±2) kyr from the late Paleocene to the early Eocene, suggesting a progressively weaker coupling of climate and the carbon-cycle with time. The higher amplitude 405-kyr cycles in the latest Paleocene are associated with changes in bottom water temperature of 2–4 °C, while the most prominent 100 kyr-paced cycles can be accompanied by changes of up to 1.5 °C. Comparison of the 1262 record with a lower resolution, but orbitally-tuned benthic record for Site 1209 in the Pacific allows for verification of key features of the benthic isotope records which are global in scale including a key warming step at 57.7 Ma.Thanks to Alexis Kersey for picking foraminifera and assisting with sample processing (at UCSC), and to Walker Weir, Alejandro Aguilar, and Phillip Staudigel for lab assistance, and to Dyke Andreasen and Chih-Ting Hsieh for stable-isotope support (UCSC). Thanks to Barbara Donner (MARUM) for coordinating foraminifera picking, and to Monika Segl and her team (MARUM) for stable-isotope analyses. We thank Roy Wilkens (Hawaii) for core images analysis. Sediment samples were supplied by the Ocean Drilling Program. Funding for this project was provided by NSF grant (grant number EAR-0628719) to J.Z. and DFG grants (RO 1113/2 through RO 1113/4) to U.R

Data report: depths of Site U1553 off-splice data adjusted to the Site U1553 splice, IODP Expedition 378

A near-complete sedimentary sequence was spliced together for the upper part of International Ocean Discovery Program (IODP) Holes U1553A, U1553B, and U1553E. Poor core recovery precluded a complete splice for the deeper section cored in Holes U1553C and U1553D. The history of Deep Sea Drilling Project Site 277, which was cored nearby, suggests that the Site U1553 splice will be heavily sampled and that eventually samples will be taken from intervals of core that are not included in the splice (i.e., off-splice). Although the depths of all cores have been shifted to a common scale during the splicing process by aligning significant features shared by cores from the different holes, core disturbance and natural variability often lead to misalignment between features in the splice and the same features in off-splice data. To remedy this problem for future sampling, data from off-splice intervals are squeezed or stretched to match spliced intervals using a set of tie points between the splice and off-splice data. The difference in depths can be significant when considering sedimentation rates and orbital periods of precession, obliquity, and eccentricity and sometimes even change the phase relationship compared to the splice. Results are presented as tables of tie points between each hole and the splice that can be used to interpolate the proper splice depth of off-splice samples

Data report: splice adjustment for Site U1553

Postcruise examination of the data splice for International Ocean Discovery Program Expedition 378 Site U1553, in light of new X-ray fluorescence data, revealed three cores from Hole U1553E that were misaligned. These cores have been shifted to fill in some gaps in the original splice

Uniform climate development between the subtropical and subpolar Northeast Atlantic across marine isotope stage 11

Proxy records from a core site off Northwest Africa were generated and compared with data from the subpolar Northeast Atlantic to unravel some main climatic features of interglacial marine isotope stage (MIS) 11 (423–362 ka). The records point to an almost 25 kyr lasting full interglacial period during stage 11 that was preceded by a considerably long glacial-interglacial transition (Termination V). Off NW Africa, a strong reduction of terrestrially derived iron input is noted after 420 ka suggesting a pronounced increase in continental humidity and vegetation cover over Northwest Africa. In analogy to the Holocene climate of the region, this early wet phase of MIS 11 was likely associated with enhanced influence of the West African monsoon system on the Saharan-Sahel region which led to both a reduction in trade wind intensity off NW Africa and the formation of sapropel S11 in the Mediterranean Sea. A detailed comparison with data from the subpolar North Atlantic indicates a remarkable coherent timing for the main environmental changes in both regions giving evidence for strong interglacial climate connection between the low and high latitude North Atlantic. Although our records of MIS 11 compare well with the Holocene in terms of some major climate characteristics there are distinct differences in the temporal evolution of each peak warm interval. This suggests that care should be taken when using MIS 11 as analogue to forecast future interglacial conditions

High resolution cyclostratigraphy of the early Eocene – new insights into the origin of the Cenozoic cooling trend

Here we present a high-resolution cyclostratigraphy based on X-ray fluorescence (XRF) core scanning data from a new record retrieved from the tropical western Atlantic (Demerara Rise, ODP Leg 207, Site 1258). The Eocene sediments from ODP Site 1258 cover magnetochrons C20 to C24 and show well developed cycles. This record includes the missing interval for reevaluating the early Eocene part of the Geomagnetic Polarity Time Scale (GPTS), also providing key aspects for reconstructing high-resolution climate variability during the Early Eocene Climatic Optimum (EECO). Detailed spectral analysis demonstrates that early Eocene sedimentary cycles are characterized by precession frequencies modulated by short (100 kyr) and long (405 kyr) eccentricity with a generally minor obliquity component. Counting of both the precession and eccentricity cycles results in revised estimates for the duration of magnetochrons C21r through C24n. Our cyclostratigraphic framework also corroborates that the geochronology of the Eocene Green River Formation (Wyoming, USA) is still questionable mainly due to the uncertain correlation of the "Sixth tuff" to the GPTS. <br><br> Right at the onset of the long-term Cenozoic cooling trend the dominant eccentricity-modulated precession cycles of ODP Site 1258 are interrupted by strong obliquity cycles for a period of ~800 kyr in the middle of magnetochron C22r. These distinct obliquity cycles at this low latitude site point to (1) a high-latitude driving mechanism on global climate variability from 50.1 to 49.4 Ma, and (2) seem to coincide with a significant drop in atmospheric CO<sub>2</sub> concentration below a critical threshold between 2- and 3-times the pre-industrial level (PAL). The here newly identified orbital configuration of low eccentricity in combination with high obliquity amplitudes during this ~800-kyr period and the crossing of a critical <i>p</i>CO<sub>2</sub> threshold may have led to the formation of the first ephemeral ice sheet on Antarctica as early as ~50 Ma ago

Astronomical calibration of the geological timescale: closing the middle Eocene gap

To explore cause and consequences of past climate change, very accurate age models such as those provided by the astronomical timescale (ATS) are needed. Beyond 40 million years the accuracy of the ATS critically depends on the correctness of orbital models and radioisotopic dating techniques. Discrepancies in the age dating of sedimentary successions and the lack of suitable records spanning the middle Eocene have prevented development of a continuous astronomically calibrated geological timescale for the entire Cenozoic Era. We now solve this problem by constructing an independent astrochronological stratigraphy based on Earth's stable 405 kyr eccentricity cycle between 41 and 48 million years ago (Ma) with new data from deep-sea sedimentary sequences in the South Atlantic Ocean. This new link completes the Paleogene astronomical timescale and confirms the intercalibration of radioisotopic and astronomical dating methods back through the Paleocene–Eocene Thermal Maximum (PETM, 55.930 Ma) and the Cretaceous–Paleogene boundary (66.022 Ma). Coupling of the Paleogene 405 kyr cyclostratigraphic frameworks across the middle Eocene further paves the way for extending the ATS into the Mesozoic

Data report: IODP Expedition 339 Site U1391: an improved splice and preliminary age model on the basis of XRF data

With the objective to reconstruct past primary production on the southwest Portuguese margin at orbital and millennial timescales through the Pleistocene, we chose to compare two sites recovered during Expedition 339 at similar latitude but different distances to the coast (Sites U1385 and U1391). Site U1385 has a well-established chronology, but for Site U1391 no isotope data are yet available. In order to develop a correct chronology without isotope data, we performed XRF analysis of cores from Holes U1391A and U1391B between 76.51 and 205.77 mbsf. With these data, in particular using log(Ca/Ti) and log(Ca/Fe), it was possible to enhance the Site U1391 splice and to establish an age model based on the correlation to the well-defined and robust Site U1385 age model.UID/Multi/04326/2019;info:eu-repo/semantics/publishedVersio

Orbitally tuned timescale and astronomical forcing in the middle Eocene to early Oligocene

Deciphering the driving mechanisms of Earth system processes, including the climate dynamics expressed as paleoceanographic events, requires a complete, continuous, and high-resolution stratigraphy that is very accurately dated. In this study, a robust astronomically calibrated age model was constructed for the middle Eocene to early Oligocene interval (31–43 Ma) in order to permit more detailed study of the exceptional climatic events that occurred during this time, including the middle Eocene climate optimum and the Eocene–Oligocene transition. A goal of this effort is to accurately date the middle Eocene to early Oligocene composite section cored during the Pacific Equatorial Age Transect (PEAT, IODP Exp. 320/321). The stratigraphic framework for the new timescale is based on the identification of the stable long eccentricity cycle in published and new high-resolution records encompassing bulk and benthic stable isotope, calibrated XRF core scanning, and magnetostratigraphic data from ODP Sites 171B-1052, 189-1172, 199-1218, and 207-1260 as well as IODP Sites 320-U1333, and 320-U1334 spanning magnetic polarity Chrons C12n to C20n. Subsequently orbital tuning of the records to the La2011 orbital solution was conducted. The resulting new timescale revises and refines the existing orbitally tuned age model and the geomagnetic polarity timescale from 31 to 43 Ma. The newly defined absolute age for the Eocene–Oligocene boundary validates the astronomical tuned age of 33.89 Ma identified at the Massignano, Italy, global stratotype section and point. The compilation of geochemical records of climate-controlled variability in sedimentation through the middle-to-late Eocene and early Oligocene demonstrates strong power in the eccentricity band that is readily tuned to the latest astronomical solution. Obliquity driven cyclicity is only apparent during 2.4 myr eccentricity cycle minima around 35.5, 38.3, and 40.1 Ma

Deglacial Variability in Okhotsk Sea Intermediate Water Ventilation and Biogeochemistry: Implications for North Pacific Nutrient Supply and Productivity

Highlights • Multi-proxy, multi-site reconstruction of Okhotsk Sea palaeo-productivity and mid-depth ventilation changes from 8 to 18 ka. • Link between hinterland river discharge and downstream Okhotsk Sea Intermediate Water (OSIW) ventilation/nutrient signatures. • Surplus Fe, Si(OH)4 export in OSIW during Bølling-Allerød to pelagic Pacific supported transient nutrient-replete conditions. • Subarctic and subtropical Pacific gyres disconnected during Bølling-Allerød, with restricted OSIW flow to lower latitudes. • Deglacial OSIW export and mid-depth Pacific biogeochemistry modulate millennial-scale regional CO2 source/sink conditions. The modern North Pacific plays a critical role in marine biogeochemical cycles, as an oceanic sink of CO2 and by bearing some of the most productive and least oxygenated waters of the World Ocean. The capacity to sequester CO2 is limited by efficient nutrient supply to the mixed layer, particularly from deeper water masses in the Pacific's subarctic and marginal seas. The region is in addition only weakly ventilated by North Pacific Intermediate Water (NPIW), which receives its characteristics from Okhotsk Sea Intermediate Water (OSIW). Here, we present reconstructions of intermediate water ventilation and productivity variations in the Okhotsk Sea that cover the last glacial termination between eight and 18 ka, based on a set of high-resolution sediment cores from sites directly downstream of OSIW formation. In a multi-proxy approach, we use total organic carbon (TOC), chlorin, biogenic opal, and CaCO3 concentrations as indicators for biological productivity. C/N ratios and XRF scanning-derived elemental ratios (Si/K and Fe/K), as well as chlorophycean algae counts document changes in Amur freshwater and sediment discharge that condition the OSIW. Stable carbon isotopes of epi- and shallow endobenthic foraminifera, in combination with 14C analyses of benthic and planktic foraminifera imply decreases in OSIW oxygenation during deglacial warm phases from c. 14.7 to 13 ka (Bølling-Allerød) and c. 11.4 to 9 ka (Preboreal). No concomitant decreases in Okhotsk Sea benthic-planktic ventilation ages are observed, in contrast to nearby, but southerly locations on the Japan continental margin. We attribute Okhotsk Sea mid-depth oxygenation decreases in times of enhanced organic matter supply to maxima in remineralization within OSIW, in line with multi-proxy evidence for maxima in primary productivity and supply of organic matter. Sedimentary C/N and Fe/K ratios indicate more effective entrainment of nutrients into OSIW and thus an increased nutrient load of OSIW during deglacial warm periods. Correlation of palynological and sedimentological evidence from our sites with hinterland reference records suggests that millennial-scale changes in OSIW oxygen and nutrient concentrations were largely influenced by fluvial freshwater runoff maxima from the Amur, caused by a deglacial northeastward propagation of the East Asian Summer Monsoon that increased precipitation and temperatures, in conjunction with melting of permafrost in the Amur catchment area. We suggest that OSIW ventilation minima and the high lateral supply of nutrients and organic matter during the Allerød and Preboreal are mechanistically linked to concurrent maxima in nutrient utilization and biological productivity in the subpolar Northwest Pacific. In this scenario, increased export of nutrients from the Okhotsk Sea during deglacial warm phases supported subarctic Pacific shifts from generally Fe-limiting conditions to transient nutrient-replete regimes through enhanced advection of mid-depth nutrient- and Fe-rich OSIW into the upper ocean. This mechanism may have moderated the role of the subarctic Pacific in the deglacial CO2 rise on millennial timescales by combining the upwelling of old carbon-rich waters with a transient delivery of mid-depth-derived bio-available Fe and silicate

Southern ocean warming, sea level and hydrological change during the Paleocene-Eocene thermal maximum

A brief (~150 kyr) period of widespread global average surface warming marks the transition between the Paleocene and Eocene epochs, ~56 million years ago. This so-called "Paleocene-Eocene thermal maximum" (PETM) is associated with the massive injection of <sup>13</sup>C-depleted carbon, reflected in a negative carbon isotope excursion (CIE). Biotic responses include a global abundance peak (acme) of the subtropical dinoflagellate <i>Apectodinium</i>. Here we identify the PETM in a marine sedimentary sequence deposited on the East Tasman Plateau at Ocean Drilling Program (ODP) Site 1172 and show, based on the organic paleothermometer TEX<sub>86</sub>, that southwest Pacific sea surface temperatures increased from ~26 °C to ~33°C during the PETM. Such temperatures before, during and after the PETM are >10 °C warmer than predicted by paleoclimate model simulations for this latitude. In part, this discrepancy may be explained by potential seasonal biases in the TEX<sub>86</sub> proxy in polar oceans. Additionally, the data suggest that not only Arctic, but also Antarctic temperatures may be underestimated in simulations of ancient greenhouse climates by current generation fully coupled climate models. An early influx of abundant <i>Apectodinium</i> confirms that environmental change preceded the CIE on a global scale. Organic dinoflagellate cyst assemblages suggest a local decrease in the amount of river run off reaching the core site during the PETM, possibly in concert with eustatic rise. Moreover, the assemblages suggest changes in seasonality of the regional hydrological system and storm activity. Finally, significant variation in dinoflagellate cyst assemblages during the PETM indicates that southwest Pacific climates varied significantly over time scales of 10<sup>3</sup> – 10<sup>4</sup> years during this event, a finding comparable to similar studies of PETM successions from the New Jersey Shelf