Testing the impact of diagenesis on the delta O-18 and delta C-13 of benthic foraminiferal calcite from a sediment burial depth transect in the equatorial Pacific

Stable oxygen and carbon isotope (δ18O and δ13C) values measured in foraminiferal calcite are one of the primary tools used in paleoceanography. Diagenetic recrystallisation of foraminiferal calcite can act to reset primary isotopic values but its effects are typically poorly quantified. Here we test the impact of early stage diagenesis on stable isotope records generated from a suite of drill sites in the equatorial Pacific Ocean recovered during Ocean Drilling Program (ODP) Leg 199 and Integrated Ocean Drilling Program (IODP) Expedition 320. Our selected sites form paleowater- and burial-depth transects, with excellent stratigraphic control allowing us to confidently correlate our records. We observe large inter-site differences in the preservation state of benthic foraminiferal calcite, implying very different recrystallisation histories, but negligible inter-site offsets in benthic δ18O and δ13C values. We infer that diagenetic alteration of benthic foraminiferal calcite (in sedimentary oozes) must predominantly occur at shallow burial depths (<100 m) where offsets in both the temperature and isotopic composition of waters in which the foraminifera calcified and pore-waters in which diagenesis occurs are small. Our results suggest that even extensive recrystallisation of benthic foraminiferal calcite results in minimal shifts from primary δ18O and δ13C values. This finding supports the long-held suspicion that diagenetic alteration of foraminiferal calcite is less problematic in benthic than in planktic foraminifera and that in deep–sea sediments routinely employed for palaeoceanographic studies benthic foraminifera are robust recorders of stable isotope values in the fossil record

A lower to middle Eocene astrochronology for the Mentelle Basin (Australia) and its implications for the geologic time scale

The geologic time scale for the Cenozoic Era has been notably improved over the last decades by virtue of integrated stratigraphy, combining high-resolution astrochronologies, biostratigraphy and magnetostratigraphy with high-precision radioisotopic dates. However, the middle Eocene remains a weak link. The so-called “Eocene time scale gap” reflects the scarcity of suitable study sections with clear astronomically-forced variations in carbonate content, primarily because large parts of the oceans were starved of carbonate during the Eocene greenhouse. International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). The sequence consists of nannofossil chalk and exhibits rhythmic clay content variability. Here, we show that IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for our U1514 astrochronology. We present a 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity. We supplement the high-resolution XRF data with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM – ca. 56 Ma) into the middle Eocene (ca. 40 Ma). Our early Eocene astrochronology corroborates existing chronologies based on deep-sea sites and Italian land sections. For the middle Eocene, the sedimentological record at U1514 provides a single-site geochemical backbone and thus offers a further step towards a fully integrated Cenozoic geologic time scale at orbital resolution

Global mean surface temperature and climate sensitivity of the early Eocene Climatic Optimum (EECO), Paleocene–Eocene Thermal Maximum (PETM), and latest Paleocene

Accurate estimates of past global mean surface temperature (GMST) help to contextualise future climate change and are required to estimate the sensitivity of the climate system to CO2 forcing through Earth's history. Previous GMST estimates for the latest Paleocene and early Eocene (∼57 to 48 million years ago) span a wide range (∼9 to 23 ∘C higher than pre-industrial) and prevent an accurate assessment of climate sensitivity during this extreme greenhouse climate interval. Using the most recent data compilations, we employ a multi-method experimental framework to calculate GMST during the three DeepMIP target intervals: (1) the latest Paleocene (∼57 Ma), (2) the Paleocene–Eocene Thermal Maximum (PETM; 56 Ma), and (3) the early Eocene Climatic Optimum (EECO; 53.3 to 49.1 Ma). Using six different methodologies, we find that the average GMST estimate (66 % confidence) during the latest Paleocene, PETM, and EECO was 26.3 ∘C (22.3 to 28.3 ∘C), 31.6 ∘C (27.2 to 34.5 ∘C), and 27.0 ∘C (23.2 to 29.7 ∘C), respectively. GMST estimates from the EECO are ∼10 to 16 ∘C warmer than pre-industrial, higher than the estimate given by the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (9 to 14 ∘C higher than pre-industrial). Leveraging the large “signal” associated with these extreme warm climates, we combine estimates of GMST and CO2 from the latest Paleocene, PETM, and EECO to calculate gross estimates of the average climate sensitivity between the early Paleogene and today. We demonstrate that “bulk” equilibrium climate sensitivity (ECS; 66 % confidence) during the latest Paleocene, PETM, and EECO is 4.5 ∘C (2.4 to 6.8 ∘C), 3.6 ∘C (2.3 to 4.7 ∘C), and 3.1 ∘C (1.8 to 4.4 ∘C) per doubling of CO2. These values are generally similar to those assessed by the IPCC (1.5 to 4.5 ∘C per doubling CO2) but appear incompatible with low ECS values (<1.5 per doubling CO2)

Temporal variability in foraminiferal morphology and geochemistry at the West Antarctic Peninsula: a sediment trap study

The West Antarctic Peninsula (WAP) exhibits strong spatial and temporal oceanographic variability, resulting in highly heterogeneous biological productivity. Calcifying organisms that live in the waters off the WAP respond to temporal and spatial variations in ocean temperature and chemistry. These marine calcifiers are potentially threatened by regional climate change with waters already naturally close to carbonate undersaturation. Future projections of carbonate production in the Southern Ocean are challenging due to the lack of historical data collection and complex, decadal climate variability. Here we present a 6-year-long record of the shell fluxes, morphology and stable isotope variability of the polar planktic foraminifera Neogloboquadrina pachyderma (sensu stricto) from near Palmer Station, Antarctica. This species is fundamental to Southern Ocean planktic carbonate production as it is one of the very few planktic foraminifer species adapted to the marine polar environments. We use these new data to obtain insights into its ecology and to derive a robust assessment of the response of this polar species to environmental change. Morphology and stable isotope composition reveal the presence of different growth stages within this tightly defined species. Inter- and intra-annual variability of foraminiferal flux and size is evident and driven by a combination of environmental forcing parameters, most importantly food availability, temperature and sea ice duration and extent. Foraminiferal growth occurs throughout the austral year and is influenced by environmental change, a large portion of which is driven by the Southern Annular Mode and El Niño–Southern Oscillation. A distinct seasonal production is observed, with the highest shell fluxes during the warmest and most productive months of the year. The sensitivity of calcifying foraminifera to environmental variability in this region, from weeks to decades, has implications both for their response to future climatic change and for their use as palaeoclimate indicators. A longer ice-free season could increase carbonate production in this region at least while carbonate saturation is still high enough to allow for thick tests to grow

Coupled evolution of temperature and carbonate chemistry during the Paleocene–Eocene; new trace element records from the low latitude Indian Ocean

This is the final version. Available on open access from Elsevier via the DOI in this recordThe early Paleogene represents the most recent interval in Earth’s history characterized by global greenhouse warmth on multi-million year timescales, yet our understanding of long-term climate and carbon cycle evolution in the low latitudes, and in particular the Indian Ocean, remains very poorly constrained. Here we present the first long-term sub-eccentricity-resolution stable isotope (δ13 30 C and δ 18 O) and trace element (Mg/Ca and B/Ca) records spanning the late Paleocene–early Eocene (~58– 53 Ma) across a surface–deep hydrographic reconstruction of the northern Indian Ocean, resolving late Paleocene 405-kyr paced cyclicity and a portion of the PETM recovery. Our new records reveal a long-term warming of ~4–5°C at all depths in the water column, with absolute surface ocean temperatures and magnitudes of warming comparable to the low latitude Pacific. As a result of warming, we observe a long-term increase in δ 18 Osw of the mixed layer, implying an increase in net evaporation. We also observe a collapse in the temperature gradient between mixed layer- and thermocline-dwelling species from ~57–54 Ma, potentially due to either the development of a more homogeneous water column with a thicker mixed layer, or depth migration of the Morozovella in response to warming. Synchronous warming at both low and high latitudes, along with decreasing B/Ca ratios in planktic foraminifera indicating a decrease in ocean pH and/or increasing dissolved inorganic carbon, suggest that global climate was forced by rising atmospheric CO2 concentrations during this time.European Consortium for Ocean Research Drilling (ECORD)International Association of Sedimentologists (IAS)NSFNatural Environment Research Council (NERC

Episodes of intensified biological productivity in the subtropical Atlantic Ocean during the termination of the Middle Eocene Climatic Optimum (MECO)

The Middle Eocene Climatic Optimum (MECO) is an ~500 kyr interval of pronounced global warming from which the climate system recovered in <50 kyr. The deep-sea sedimentary record can provide valuable insight on the marine ecosystem response to this protracted global warming event and consequently on the ecological changes during this time. Here we present new benthic foraminiferal assemblage data from Ocean Drilling Program Site 1051 in the subtropical North Atlantic, spanning the MECO and post-MECO interval (41.1 to 39.5 Ma). We ␣nd little change in the species composition of benthic foraminiferal assemblages during the studied interval, suggesting that the rate of environmental change was gradual enough that these organisms were able to adapt. However, we identify two transient intervals associated with peak warming (higher-productivity interval (HPI)-1; 40.07–39.96 Ma) and shortly after the MECO (HPI-2; 39.68–39.55 Ma), where benthic foraminiferal accumulation rates increase by an order of magnitude. These HPIs at Site 1051 appear to coincide with intervals of strengthened productivity in the Tethys, Southern Ocean, and South Atlantic, and we suggest that an intensi␣ed hydrological cycle during the climatic warmth of the MECO was responsible for eutrophication of marine shelf and slope environments