Production and temperature sensitivity of long chain alkenones in the cultured haptophyte Pseudoisochrysis paradoxa

The alkenone unsaturation index (U<sub>37</sub><sup>K</sup> or U<sub>37</sub><sup>K′</sup>) serves as a critical tool for reconstructing temperature in marine environments. Lacustrine haptophyte algae are genetically distinct from their ubiquitous and well studied marine counterparts, and the unknown species-specific genetic imprints on long chain alkenone production by lacustrine species have hindered the widespread application of the U37<sup>K</sup> temperature proxy to lake sediment records. The haptophyte Pseudoisochrysis paradoxa produces alkenones but its U37<sup>K</sup> calibration has never been determined. It has an alkenone fingerprint abundant in tetraunsaturated alkenones, a hallmark of lacustrine environments. We present here the first calibration of the U37<sup>K</sup> index to temperature for a culture of P. paradoxa. We found that the U37<sup>K</sup> index accurately captured the alkenone response to temperature whereas the U37<sup>K′</sup> index failed to do so, with U37<sup>K′</sup> values below 0.08 projecting to two different temperature values. Our results add a fifth species-specific U37<sup>K</sup> calibration and provide another line of evidence that different haptophyte species require different U37<sup>K</sup> calibrations. The findings also highlight the necessary inclusion of the C<sub>37:4</sub> alkenone when reconstructing temperatures from P. paradoxa-derived alkenone records

Solar Resonant Diffusion Waves as a Driver of Terrestrial Climate Change

A theory is described based on resonant thermal diffusion waves in the sun that appears to explain many details of the paleotemperature record for the last 5.3 million years. These include the observed periodicities, the relative strengths of each observed cycle, and the sudden emergence in time for the 100 thousand year cycle. Other prior work suggesting a link between terrestrial paleoclimate and solar luminosity variations has not provided any specific mechanism. The particular mechanism described here has been demonstrated empirically, although not previously invoked in the solar context. The theory also lacks most of the problems associated with Milankovitch cycles.Comment: in press with The Journal of Atmospheric and Solr Terrestrial Physic

Coupled Mg/Ca and clumped isotope analyses of foraminifera provide consistent water temperatures

The reliable determination of past seawater temperature is fundamental to paleoclimate studies. We test the robustness of two paleotemperature proxies by combining Mg/Ca and clumped isotopes (Δ47) on the same specimens of core top planktonic foraminifera. The strength of this approach is that Mg/Ca and Δ47 are measured on the same specimens of foraminifera, thereby providing two independent estimates of temperature. This replication constitutes a rigorous test of individual methods with the advantage that the same approach can be applied to fossil specimens. Aliquots for Mg/Ca and clumped analyses are treated in the same manner following a modified cleaning procedure of foraminifera for trace element and isotopic analyses. We analysed eight species of planktonic foraminifera from coretop samples over a wide range of temperatures from 2 to 29°C. We provide a new clumped isotope temperature calibrations using subaqueous cave carbonates, which is consistent with recent studies. Tandem Mg/Ca–Δ47 results follow an exponential curve as predicted by temperature calibration equations. Observed deviations from the predicted Mg/Ca-Δ47 relationship are attributed to the effects of Fe-Mn oxide coatings, contamination, or dissolution of foraminiferal tests. This coupled approach provides a high degree of confidence in temperature estimates when Mg/Ca and Δ47 yield concordant results, and can be used to infer the past δ18O of seawater (δ18Osw) for paleoclimate studies

Stable and Clumped Isotope Characterization of Authigenic Carbonates in Methane Cold Seep Environments

Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ¹³C, δ¹⁸O, δ⁴⁴Ca, and Δ₄₇) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ₄₇-based paleotemperature archive. The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ¹³C_(VPDB) values (−30 to −47‰), high δ⁴⁴Ca_(SW) values (−0.4 to −0.8‰), and Δ₄₇-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ¹³C_(VPDB) values (−18 to −58‰), lower δ⁴⁴Ca_(SW) values (−0.8 to −1.6‰) and a larger range in Δ₄₇-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values. The range in apparent temperatures as well as δ⁴⁴Ca_(SW) values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO₂ degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions

Changes in the seawater salinity-oxygen isotope relation between last glacial and present: sediment core data and OGCM modelling

The presently available paleotemperature data implies large ice-free areas in the Greenland- Iceland-Norwegian Seas during the Last Glacial Maximum 21 600 yr BP. From these temperatures and the independent measurements of oxygen isotope ratios of fossil foraminiferal shells, glacial sea surface salinities could be computed, if the glacial relation between salinity and water isotope ratio was known. For this study, a three-dimensional numerical ocean circulation model was employed to investigate the possible shape of this still not precisely known relation, and to reconstruct a physically consistent scenario of the northern North Atlantic for the glacial summer. This scenario turned out to be quite similar to modern winter conditions, whereas the required salinity vs. oxygen isotope relation of this time must have been very different from its modern counterpart

Oxygen isotopic paleotemperatures across the Runangan-Whaingaroan (Eocene-Oligocene) boundary in a New Zealand shelf sequence

Oxygen isotopic compositions of the tests of mainly benthic foraminifera, from sections of conformable Late Eocene (Runangan) to Early Oligocene (Whaingaroan) shelf mudstones, at both Cape Foulwind and Port Elizabeth, western South Island, indicate that shelf sea paleotemperatures followed the global open-ocean trend towards a Paleogene minimum near the Eocene-Oligocene boundary. Throughout the latest Eocene, temperatures declined steadily by 3°C, showed a temporary minor warming at the Eocenc-Oligocene boundary, dropped sharply by 2°C in the Early Oligocene, and ameliorated significantly later in the Early Oligocene. The qualitative temperature trends for New Zealand shelf waters at this time are similar to those inferred from earlier paleontologic syntheses and limited oxygen isotopic work, but involve a range of temperatures within the warm and cool temperate climatic zones and an absolute temperature depression across the Eocene-Oligocene boundary of only 5°C from about 17 to 12°C. Results are consistent with isotopic paleotemperatures determined from deep-sea sediment cores south of New Zealand where the cooling is inferred to mark the onset of production of Antarctic bottom waters at near-freezing temperatures

Alkenone producers during late Oligocene-early Miocene revisited

This study investigates ancient alkenone producers among the late Oligocene–early Miocene coccolithophores recorded at Deep Sea Drilling Project (DSDP) Site 516. Contrary to common assumptions, Reticulofenestra was not the most important alkenone producer throughout the studied time interval. The comparison between coccolith species-specific absolute abundances and alkenone contents in the same sedimentary samples shows that Cyclicargolithus abundances explain 40% of the total variance of alkenone concentration and that the species Cyclicargolithus floridanus was a major alkenone producer, although other related taxa may have also contributed to the alkenone production at DSDP Site 516. The distribution of the different alkenone isomers (MeC37:2, EtC38:2, and MeC38:2) remained unchanged across distinct changes in species composition, suggesting similar diunsaturated alkenone compositions within the Noelaerhabdaceae family during the late Oligocene–early Miocene. However, the overall larger cell size of Cyclicargolithus may have implications for the alkenone-based reconstruction of past partial pressure of CO2. Our results underscore the importance of a careful evaluation of the most likely alkenone producers for periods (>1.85 Ma) predating the first occurrence of contemporary alkenone producers (i.e., Emiliania huxleyi and Gephyrocapsa oceanica)