ALBERT

Description: Marine calcifying eukaryotic phytoplankton (coccolithophores) is a major contributor to the pelagic production of CaCO3 and plays an important role in the biogeochemical cycles of C, Ca and other divalent cations present in the crystal structure of calcite. The geochemical signature of coccolithophore calcite is used as palaeoproxy to reconstruct past environmental conditions and to understand the underlying physiological mechanisms (vital effects) and precipitation kinetics. Here, we present the stable Sr isotope fractionation between seawater and calcite (Δ88/86Sr) of laboratory cultured coccolithophores in individual dependence of temperature and seawater carbonate chemistry. Coccolithophores were cultured within a temperature and a pCO2 range from 10 to 25°C and from 175 to 1,240 μatm, respectively. Both environmental drivers induced a significant linear increase in coccolith stable Sr isotope fractionation. The temperature correlation at constant pCO2 for Emiliania huxleyi and Coccolithus braarudii is expressed as Δ88/86Sr = −7.611 × 10−3 T + 0.0061. The relation of Δ88/86Sr to pCO2 was tested in Emiliania huxleyi at 10 and 20°C and resulted in Δ88/86Sr = −5.394 × 10−5 pCO2 – 0.0920 and Δ88/86Sr = −5.742 × 10−5 pCO2 – 0.1351, respectively. No consistent relationship was found between coccolith Δ88/86Sr and cellular physiology impeding a direct application of fossil coccolith Δ88/86Sr as coccolithophore productivity proxy. An overall significant correlation was detected between the elemental distribution coefficient (DSr) and Δ88/86Sr similar to inorganic calcite with a physiologically induced offset. Our observations indicate (i) that temperature and pCO2 induce specific effects on coccolith Δ88/86Sr values and (ii) that strontium elemental ratios and stable isotope fractionation are mainly controlled by precipitation kinetics when embedded into the crystal lattice and subject to vital effects during the transmembrane transport from seawater to the site of calcification. These results provide an important step to develop a coccolith Δ88/86Sr palaeoproxy complementing the existing toolbox of palaeoceanography.

Description: The recrystallisation (dissolution–precipitation) of carbonate sediments has been successfully modelled to explain profiles of pore water Sr concentration and radiogenic Sr isotope composition at different locations of the global ocean. However, there have been few systematic studies trying to better understand the relative importance of factors influencing the variability of carbonate recrystallisation. Here we present results from a multi-component study of recrystallisation in sediments from the Integrated Ocean Drilling Program (IODP) Expedition 320/321 Pacific Equatorial Age Transect (PEAT), where sediments of similar initial composition have been subjected to different diagenetic histories. The PEAT sites investigated exhibit variable pore water Sr concentrations gradients with the largest gradients in the youngest sites. Radiogenic Sr isotopes suggest recrystallisation was relative rapid, consistent with modelling of other sediment columns, as the 87Sr/86Sr ratios are indistinguishable (within 2σ uncertainties) from contemporaneous seawater 87Sr/86Sr ratios. Bulk carbonate leachates and associated pore waters of Site U1336 have lower 87Sr/86Sr ratios than contemporaneous seawater in sediments older than 20.2 Ma most likely resulting from the upward diffusion of Sr from older recrystallised carbonates. It seems that recrystallisation at Site U1336 may still be on-going at depths below 102.5 rmcd (revised metres composite depth) suggesting a late phase of recrystallisation. Furthermore, the lower Sr/Ca ratios of bulk carbonates of Site U1336 compared to the other PEAT sites suggest more extensive diagenetic alteration as less Sr is incorporated into secondary calcite. Compared to the other PEAT sites, U1336 has an inferred greater thermal gradient and a higher carbonate content. The enhanced thermal gradient seems to have made these sediments more reactive and enhanced recrystallisation. In this study we investigate stable Sr isotopes from carbonate-rich deep sea sediments for the first time. Pore water δ88/86Sr increases with depth (from 0.428‰ to values reaching up to 0.700‰) at Site U1336 documenting an isotope fractionation process during recrystallisation. Secondary calcite preferentially incorporates the lighter Sr isotope (86Sr) leaving pore waters isotopically heavy. The δ88/86Sr values of the carbonates themselves show more uniform values with no detectable change with depth. Carbonates have a much higher Sr content and total Sr inventory than the pore waters meaning pore waters are much more sensitive to fractionation processes than the carbonates. The δ88/86Sr results indicate that pore water stable Sr isotopes have the potential to indicate the recrystallisation of carbonate sediments.

Description: In the light of increasing interest in the role of the world’s oceans in future global change, it is important to understand the underlying processes that were responsible for paleoceanographic changes in the geological past. Previous studies working on the geochemistry of ancient marine deposits agree that the seawater chemistry must have been changed throughout the Phanerozoic Eon. In particular, concurrent long-term changes in radiogenic and stable isotope systems, element ratios and concentrations and their correlation with sea-level changes, climate reconstructions, and global mass extinctions suggests common causative mechanisms. However, the geological processes being responsible for the observed changes are still being debated. Specifically, the role of mid ocean spreading rates, dolomitization and sea-level changes are thought to play a major role in paleo-seawater chemistry of major and trace elements. Within this study the first Phanerozoic stable strontium (Sr) isotope seawater record (δ(88/86)Sr-sw) is reconstructed, which is sensitive to imbalances in the Sr input and output fluxes. In a consequent model approach, the radiogenic Sr isotope record (87-Sr/86-Sr)sw and δ(88/86)Sr-sw are used to constrain the marine Phanerozoic Sr budget. On long timescales (~200Myr periodicity), δ(88/86)Sr-sw and modelled Sr carbonate burial rates (F(Sr)carb) follow times of proposed „aragonite seas“ and „calcite seas“, implying that the dominant carbonate mineralogy has an important effect on Sr burial rates. On shorter timescales, minima and maxima in F(Sr)carb are partly correlated to ocean anoxia and glaciations and related sea-level low stands, implying the importance of continental carbonate shelf weathering to the marine Sr budget. In particular, enduring high carbonate burial rates for ~21Myr could be related to seawater anoxia during the end-Permian mass extinctions. Here, bacterial sulphate reduction rates led to toxic and high alkaline deep waters that were intermittently upwelled to the surface ocean, causing massive carbonate precipitation on the seafloor as well as the largest biogeochemical crisis in the Phanerozoic Eon. Ultimately, insights from changes in δ(88/86)Sr-sw significantly improved our understanding of long-term changes in seawater chemistry and the relation of carbonate-related Sr fluxes to sea-level changes, mass extinctions, and global anoxia.

Description: We present strontium (Sr) isotope ratios that, unlike traditional 87Sr/86Sr data, are not normalized to a fixed 88Sr/86Sr ratio of 8.375209 (defined as δ88/86Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87Sr–84Sr double spike. This technique yields two independent ratios for 87Sr/86Sr and 88Sr/86Sr that are reported as (87Sr/86Sr*) and (δ88/86Sr), respectively. The difference between the traditional radiogenic (87Sr/86Sr normalized to 88Sr/86Sr = 8.375209) and the new 87Sr/86Sr* values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater ((87Sr/86Sr*, δ88/86Sr)Seawater) and modern marine biogenic carbonates ((87Sr/86Sr*, δ88/86Sr)Carbonates) with the corresponding values of river waters ((87Sr/86Sr*, δ88/86Sr)River) and hydrothermal solutions ((87Sr/86Sr*, δ88/86Sr)HydEnd) in a triple isotope plot. The measured (87Sr/86Sr*, δ88/86Sr)River values of selected rivers that together account for not, vert, similar18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114(8), 0.315(8)‰). The average (87Sr/86Sr*, δ88/86Sr)HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)‰). In contrast, the (87Sr/86Sr*, δ88/86Sr)Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)‰). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)‰). The difference between the estimated (87Sr/86Sr*, δ88/86Sr)input and (87Sr/86Sr*, δ88/86Sr)output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10–30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely “glacial” source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the “weathering peak” hypothesis that invokes enhanced Sr inputs resulting from weathering of post-glacial exposure of abundant fine-grained material.