Clumped isotope thermometry applied to carbonate fossils is a promising technique to derive independent and accurate reconstructions of absolute ocean temperatures, a key parameter in understanding past Earth Climate Sensitivity. Other more commonly used temperature proxies have several disadvantages, including requiring assumptions of seawater chemistry compositions (e.g. foraminifera Mg/Ca and δ18O), or being based on empirical correlations without a complete understanding of its controlling mechanisms (e.g. TEX86 and Uk'37). Conversely, clumped isotope thermometry is based on thermodynamics, and is independent from seawater chemistry. Here we present clumped isotopes (Δ47) in coccolith separations from globally distributed Holocene core tops, a monospecific Coccolithus pelagicus sediment trap in the Iceland Sea, downcore sediments from the North Atlantic during the last 16 Ma, and downcore sediments from tropical (Equatorial Pacific) and high latitudes (South Tasman Rise) spanning the Cenozoic. Calcification temperatures of the sediment trap agree with satellite derived temperatures, further supporting a lack of or small vital effects in coccolith clumped isotopes. Temperatures derived from Δ47 of tropical Holocene coccoliths are colder than modern Sea Surface Temperatures (SSTs). This suggests that coccolithophores may inhabit deeper than surface waters in these areas, which if proven to be true, would have implications for how other proxies, such as Uk'37, are calibrated to SSTs. At higher latitudes, calcification temperatures from Holocene coccolith separations are more similar to SSTs, and we suggest they are indicators of mixed layer depth temperatures in these regions.Pure coccoliths from the North Atlantic during the last 16 Ma show Δ47-derived temperatures that are 10 °C colder than those derived with alkenones from the same samples. This suggests a modest, rather than an extreme polar amplification, which agrees better with climate models. Scanning Electron Microscopy (SEM) and trace elements show no evidence of significant recrystalization and therefore cannot explain such large differences in reconstructed temperatures with both proxies.Preliminary low resolution Δ47 calcification temperatures of pure coccolith separations from the Equatorial Pacific throughout the Cenozoic show similar trends to the overall climate pattern expected from foraminiferal δ18O, but with colder absolute values. For example, published core top Δ47 coccoliths indicate warmer temperatures compared to our 2 My sample in core U1338, and may suggest potential early recrystalization effects, different sources or strength of upwelling in the past oceans, latitudinal movement of upwelling, or depth of production. Conversely, high latitude temperatures (ODP 1170) from our youngest coccolith separation (2 My) agrees better with modern SSTs and alkenone temperatures. The general expected climatic trend is also observed in our high latitude record, although the magnitude of cooling through time is less marked compared to that shown in the Equatorial Pacific. Trace element and SEM imaging could give insights on whether there is evidence of some recrystalization, or other interfering material in the analyzed pure coccolith fractions, despite the careful separation process that limited the presence of non-coccolith carbonate. Our results show that coccolith Δ47 has the promising potential to derive reconstructions of temperatures of euphotic oceans over the Cenozoic.
<p>&#160; &#160; &#160;Reliable temperature reconstructions of the ocean are often difficult to obtain due to the limitations of widely used proxies. The application of clumped isotope thermometry to coccolith calcite, which is geographical and chronological ubiquitously distributed, and whose production is limited to the photic zone, may provide ocean&#8217;s temperature information when and where other proxies have been shown inaccurate or not applicable.</p><p>&#160; &#160; &#160;To evaluate the potential of coccolith clumped isotopes in paleoceanography we compare the temperatures derived from the fine fraction (<11&#181;m), a pure mixed coccolith fraction (2-10 &#181;m), and to a fraction of carbonate fragments from unidentified sources (<2 &#181;m), with coeval alkenone sea surface temperatures (SST) from ODP Site 982 in the North Atlantic covering the last 16 Ma. The similarity in magnitudes and trends from the <11 and 2-10 &#181;m size fractions, and trace element analysis of the <2 &#181;m size fraction, suggest that for this site and time interval, exclusion of small unrecognizable fragments is not necessary to obtain reliable temperatures. The warmer values of alkenone SSTs compared to coccolith clumped isotope-derived temperatures cannot be explained by diagenetic processes, but may be related to temperature overestimations by alkenone calibrations, which assume a warm season and/or shallow production of coccolithophores in the study area.&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;</p><p>&#160; &#160; &#160;Vital effects in coccolith clumped isotopes potentially associated to carbon limitation may also help to explain the differences in cooling magnitudes compared to the alkenone record. To further investigate vital effects in clumped isotopes, we compare calcification temperatures of three pure coccolith size fractions (3-5, 5-8, and 8-10 &#181;m), and relate them to vital effects observed in their &#948;<sup>13</sup>C and &#948;<sup> 18</sup>O. The analysis of the fine fraction of Holocene sediments (<10 or <8 &#181;m) showing a range of temperature and CO<sub>2</sub> concentrations also provide information on the potential effects of carbon availability in coccolith clumped isotopes, and suggests calcification of coccolithophores may occur in deeper habitats than those considered by alkenone calibrations. Our study shows clumped isotope thermometry applied to coccolith calcite as a promising alternative proxy for calcification temperature of coccolithophores.</p>
Reliable temperature reconstructions are necessary to improve climate reconstructions and comparisons with paleoclimate model simulations. Most existing paleotemperature proxies are based on organic and inorganic remains of marine organisms. Despite the evidence that the habitat depth of coccolithophores and other phytoplankton depend on their ability to balance light, nutrients, and grazing pressure, calibrations of proxies based on photosynthesizers often assume they live in the surface ocean. Here we present the first globally distributed dataset of core top multi-species coccolith clumped isotopes (Δ47), which show a clear latitudinal thermal gradient and demonstrate coccolith Δ47 sensitivity to temperature. The application of the most recent Δ47-temperature calibration for marine biogenic carbonates yields calcification temperatures implying deep habitats of ∼50 to ∼150 m for tropical coccolithophores, which could photosynthesize with 1-10% of surface photosynthetic active radiation (PAR) levels. Because of the uncertainties of Δ47 thermometry and of the low upper ocean temperature gradient, at well-mixed high-latitude locations, coccolith Δ47 cannot be used to reliably constrain a specific habitat depth. Nevertheless, Δ47 is a good indicator of paleotemperatures of the mixed layer. We also use coccolith Δ47 to derive the first regression relating core top coccolith Δ47 and sea surface temperatures (SST). Although this formulation cannot be considered a proper coccolith-specific Δ47 calibration, since it ignores coccolithophore's potential for calcification at depth, it facilitates comparison with temperature proxies like U37k′, which are regressed to SST, rather than production temperature.
Abstract Periodic ~400 kyr orbital scale variations in the ocean carbon cycle, manifest in indicators of deep sea dissolution and benthic 13C, have been observed throughout the Cenozoic but the driving mechanisms remain under debate. Changes in coccolithophore productivity may change the global rain ratio (Corganic: Cinorganic fluxes from ocean into sediment) and the balance of ocean carbonate system and thereby, potentially contributing to the ~400 kyr oscillation of the marine carbon cycle. Some evidence suggests that Pleistocene coccolithophore productivity was characterized by “bloom” events of high productivity coincident with the orbital benthic 13C signal. However, there is no consensus on the mechanism responsible for bloom events nor whether they were regional or global phenomena. In this study, we investigate the timing and spatial pattern of the most recent purported coccolithophore bloom event, which occurred during the Mid-Brunhes period. We find that maximum coccolithophore productivity is diachronous, peaking in the Southern Ocean sub-Antarctic zone with eccentricity minimum (~430 ka), peaking in upwelling zones some ~28 kyr later, and finally peaking in the western tropical Pacific occurred some ~80 kyr later. Simple globally homogeneous mechanisms of driving productivity such as temperature or light duration are not consistent with this pattern. Rather, we propose a dual high and low latitude control on blooms. Coincident with eccentricity minimum, increased high-latitude diatom silica consumption lowers the Si/P, leading to coccolithophorid blooms in the Southern Ocean north of the polar front. Coincident with increasing eccentricity, stronger tropical monsoons deliver higher fluvial nutrients to surface waters, increasing total (diatom and coccolithophore) productivity. Most of the tropical and subtropical locations are influenced by both processes with varying degrees, through the effect of silicic acid leakage on tropical thermocline waters and monsoon-related nutrient supply. Moreover, we propose that the high latitude processes have intensified over the Pleistocene, extending the 405 kyr carbon cycle to about 500 kyr.
Coccolithophores are calcifying marine phytoplankton whose blooms can be seen from space and play an important, yet complex, role in the global carbon cycle. On one hand, coccolithophores sequester atmospheric CO2 to the deep ocean via photosynthesis contributing to the biological pump. On the other hand, coccolithophores increase aqueous CO2 via precipitation of tiny calcite scales named coccoliths (i.e., carbonate counter pump), which are a major component of marine sediments. Coccoliths are generally in the 2-20 µm size range, and thus they can be winnowed by strong currents and transported to distal locations. Here, we show the first coccoliths radiocarbon (14C) ages and explore the influence of size-dependent coccolith sorting and transport, redistribution, and fate in marine sediments. Because the coccolith depends on the species, we have separated and 14C dated four coccolith size fractions: 8-11 µm, 5-8 µm, 3-5 µm, and 2-3 µm, in  five depth intervals on a sediment core recovered from SHAK06-5K site, off the Iberian Margin. Coccolith separation was achieved by a combination of dry sieving, microfiltration, centrifugation, and settling experiments. Energy Dispersive Spectroscopy (EDS) images of selected size fractions were used to estimate the relative contribution of coccolith and non-coccolith carbonate. A relationship between coccolith 14C age and grain size is apparent in all samples, with the smallest size class recording the youngest ages and the largest coccoliths being the oldest. The latter suggests that hydrodynamic sorting largely influences coccolith redistribution in marine sediments, where larger coccoliths result in increased mobility, as they are prone to resuspension than coccoliths in 2-3 µm size fraction that tend to show cohesive behaviour. The 14C ages of coccoliths are older than those of co-deposited planktic foraminifera, bulk organic carbon (OC), long-chain fatty acids (LCFA), and alkenones. Coccoliths within the 2-3 µm size class show 14C ages comparable to those of OC in all samples. Such a pattern indicates similar transport mechanisms for both the smallest coccoliths and OC, and that the majority of carbonate in the 2-3 µm size fraction, including the non-coccolith particles, is predominantly derived from marine primary production and thus, of biogenic origin. Our study has implications for palaeoceanographic studies using coccoliths as paleo-productivity and geochemical proxies.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)