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1. Reconstructing paleoenvironments of the Late Cretaceous Western Interior Seaway, USA, using paired triple oxygen and carbonate clumped isotope measurements

   https://doi.org/10.1130/B37543.1  

   Wostbrock, Jordan AG; Witts, James D; Gao, Yang; Peshek, Catherine; Myers, Corinne E; Henkes, Gregory; Sharp, Zachary D (July 2024, Geological Society of America Bulletin)

   Fossiliferous carbonate concretions are commonly found in sediments deposited in the Late Cretaceous Western Interior Seaway. Although concretions are diagenetic features, well-preserved fossils from within them have been instrumental in reconstructing the temperature and δ18O value of Western Interior Seaway seawater, which is essential for accurate reconstruction of Late Cretaceous climate. Here, we constrain formation conditions of Late Campanian and early Maastrichtian carbonate concretions by combining triple oxygen isotope measurements with carbonate clumped isotope paleothermometry on different carbonate phases within the concretions. We measured both fossil skeletal aragonite and sparry calcite infill from cracks and within macrofossil voids to evaluate differences between “primary” and “altered” geochemical signals. Based on the two temperature-sensitive isotope systems of the primary fossil shell aragonite, the temperature of the Western Interior Seaway was between 20 °C and 40 °C and was likely thermally stratified during the Campanian. The reconstructed δ18Oseawater values of ∼−1‰ for Campanian Western Interior Seaway waters are similar to those expected for the open ocean during greenhouse climates, while the Maastrichtian Western Interior Seaway may have been more restricted, with a δ18Oseawater value of ∼2‰, which reflects more evaporative conditions. We reconstructed the diagenetic history of the sparry infill and altered fossils using a fluid-rock mixing model. Alteration temperature, alteration fluid δ18O value, and the initial formation temperature were calculated by applying the fluid-rock mixing model to a particle swarm optimization algorithm. We found a different range of initial formation temperatures between the Campanian (25−38 °C) and Maastrichtian (9−28 °C). We also found that alteration in the presence of light meteoric fluids (δ18O ≈ −10‰) is required to explain both the sparry infill and the altered fossil isotopic values. Based on our results, both lithification and alteration of the carbonates occurred soon after burial, and light meteoric fluids support prior findings that high-topographic relief existed on the western margin of the Western Interior Seaway during the Late Cretaceous. As one of the first studies to apply these techniques in concert and across multiple mineralogical phases within samples, our results provide important constraints on paleoenvironmental conditions in an enigmatic ocean system and will improve interpretations of the overall health of ecosystems leading into the end-Cretaceous mass extinction. 

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2. Simulation of oxygen isotopes and circulation in a late Carboniferous epicontinental sea with implications for proxy records

   https://doi.org/10.1016/j.epsl.2021.116770  

   Macarewich, Sophia I.; Poulsen, Christopher J.; Montañez, Isabel P. (April 2021, Earth and Planetary Science Letters)

   Reconstructions of ancient ocean chemistry are largely based on geochemical proxies obtained from epicontinental seas. Mounting evidence suggests that these shallow inland seas were chemically distinct from the nearby open ocean, decoupling epicontinental records from broader ocean conditions. Here we use the isotope-enabled Community Earth System Model to evaluate the extent to which the oxygen isotopic composition of the late Carboniferous epicontinental sea, the North American Midcontinent Sea (NAMS), reflects the chemistry of its open-ocean sources and connect epicontinental isotope variability in the sea to large-scale ocean-atmosphere processes. Model results support estuarine-like circulation patterns demonstrated by past empirical studies and suggest that orographic runoff produced decreases in surface seawater δ18O (δ18Ow) of up to ∼3between the NAMS and the bordering ocean. Simulated sea surface temperatures are relatively constant across the sea and broadly reproduced from proxy-based δ18O paleotemperatures for which model-based values of epicontinental δ18Oware used, indicating that offshore-onshore variability in surface proxy δ18O is primarily influenced by seawater freshening. Simulated bottom water temperatures in the NAMS are also reproduced from biogenic calcite δ18O using model-based values of epicontinental δ18Ow, suggesting that benthic marine fossil δ18O is also influenced by seawater freshening and coastal upwelling. In addition, glacial-interglacial variations in nearshore seawater freshening counteract the effects of temperature on marine biogenic δ18O values, suggesting that salinity effects should be considered in δ18O-based estimates of glacioeustatic sea level change from nearshore regions of the NAMS. Our results emphasize the importance of constraining epicontinental dynamics for interpretations of marine biogenic δ18O as proxies of paleotemperature, salinity, and glacioeustasy. 

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3. Determining the source of speleothem δ18O variability from in situ measurements of seasonal and inter-annual isotope trends in precipitation, cave drip water and modern calcite from sites in the Central Peruvian Andes

   Olson, E.J. (December 2022, American Geophysical Union Conference. Chicago, IL)

   In the past decade, Huagapo and Pacupahuain Caves in the Central Peruvian Andes have become sources of speleothem oxygen isotope (δ18O) paleoclimate records. These studies identify the South American Summer Monsoon (SASM) as the main climate system controlling δ18O variability. While this interpretation is verified through inter-proxy record comparisons on millennial scales, interpretation of the high-resolution variability within these records is limited by a lack of modern proxy calibration studies at these sites. Here we present results from a modern cave monitoring study undertaken to address the controls on the δ18O values of precipitation at these sites and how surface and in-cave processes affect the δ18O value of speleothem calcite. Speleothem calcite δ18O values reflect an integrated signal of atmospheric processes (e.g., rainout, Raleigh distillation, upstream moisture recycling, changes in moisture source), evaporation and mixing during infiltration in the soil and epikarst, and in-cave processes such as degassing and evaporation. In consideration of these factors, we compare isotope trends in precipitation, cave drip water and modern farmed calcite from the two cave sites. We find that precipitationδ18O values during peak monsoon activity (January -February) shows considerable inter-annual variation with averages of -16.7‰ for 2020, -18.5‰ for 2021 and -13.8‰ in 2022. We investigate the source of this variability in regional atmospheric circulation patterns using weather station data and back trajectories. The mean annual precipitation (MAP) from outside Huagapo Cave is δ18O = -15.5+/- 6‰, while seasonal samples of drip water δ18O = -14.5+/- 1‰, are offset from MAP possibly due to evaporation during infiltration. Cave drip waterδ18O has low variability over inter-annual and seasonal timescales indicating homogenization in the epikarst. Using geochemical and sensor data (e.g. cave relative humidity, temperature, and drip rate) as inputs for a karst based forward model, we simulate modern speleothem δ18O to quantitatively assess the combined effects of hydroclimate processes integration to the isotope record. 

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4. High‐Resolution Mg/Ca and δ18O Patterns in Modern Neogloboquadrina pachyderma From the Fram Strait and Irminger Sea

   Livsey, C.M.; Kozdon, R.; Bauch, D.; Brummer, G-J A.; Jonkers, L.; Orland, I.; Hill, T. M.; Spero, H.S. (January 2020, Paleoceanography and paleoclimatology)

   Abstract Neogloboquadrina pachyderma is the dominant species of planktonic foraminifera found in polar waters and is therefore invaluable for paleoceanographic studies of the high latitudes. However, the geochemistry of this species is complicated due to the development of a thick calcite crust in its final growth stage and at greater depths within the water column. We analyzed the in situ Mg/Ca and δ18O in discrete calcite zones using laser ablation‐inductively coupled plasma‐mass spectrometry, electron probe microanalysis, and secondary ion mass spectrometry within modern N. pachyderma shells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. Here we compare shell geochemistry to the measured temperature, salinity, and δ18Osw in which the shells calcified to better understand the controls on N. pachyderma geochemical heterogeneity. We present a relationship between Mg/Ca and temperature in N. pachyderma lamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. We also document highly variable secondary ion mass spectrometry δ18O results (up to a 3.3‰ range in single shells) on plankton tow samples which we hypothesize is due to the granular texture of shell walls. Finally, we document that the δ18O of the crust and lamellar calcite of N. pachyderma from an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts in N. pachyderma δ18O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions. 

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5. Geochemical Differences Between Alive, Uncrusted and Dead, Crusted Shells of Neogloboquadrina pachyderma: Implications for Paleoreconstruction

   Hupp, Brittany; Fehrenbacher, Jennifer (September 2023, Paleoceanography and paleoclimatology an AGU journal exploring earths paleoclimate)

   Planktic foraminiferal-based trace element-calcium ratios (TE/Ca) are a cornerstone in paleoceanographic reconstructions. While TE-environment calibrations are often established through culturing experiments, shell growth in culture is not always consistent with growth in a natural setting. For example, many species of planktic foraminifera thicken their shell at the end of their life cycle, producing a distinct “gametogenic” crust. Crust is common in fossil foraminifers, however, shells grown in culture do not often develop a thick crust. Here, we investigate potential vital effects associated with the crusting process by comparing the trace element (Mg/Ca, Na/Ca, Ba/Ca, Sr/Ca, Mn/Ca, Zn/Ca) and stable isotope (δ13C, δ18O) composition of alive, fully mature, uncrusted shells to recently deceased, crusted shells of Neogloboquadrina pachyderma collected from the same plankton tows off the Oregon (USA) coast. We find that uncrusted (N = 55) shells yield significantly higher Ba/Ca, Na/Ca, Mn/Ca, and Sr/Ca than crusted (N = 66) shells, and crust calcite records significantly lower TE/Ca values for all elements examined. Isotopic mixing models suggest that the crust calcite accounts for ∼40%–70% of crusted shell volume. Comparison of foraminiferal and seawater isotopes indicate that N. pachyderma lives in the upper 90 m of the water column, and that crust formation occurs slightly deeper than their average living depth habitat. Results highlight the necessity to establish calibrations from crusted shells, as application of calibrations from TE-enriched uncrusted shells may yield attenuated or misleading paleoceanographic reconstructions. 

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