Calcium isotopes in scleractinian fossil corals since the Mesozoic: Implications for vital effects and biomineralization through time

We present a Cenozoic record of δ^(44/40)Ca from well preserved scleractinian fossil corals, as well as fossil coral δ^(44/40)Ca data from two time periods during the Mesozoic (84 and 160 Ma). To complement the coral data, we also extend existing bulk pelagic carbonate records back to ∼80 Ma. The same fossil corals used for this study were previously shown to be excellently preserved, and to be faithful archives of past seawater Mg/Ca and Sr/Ca since ∼200 Ma (Gothmann et al., 2015). We find that the δ^(44/40)Ca compositions of bulk pelagic carbonates from ODP Site 807 (Ontong Java Plateau) and DSDP Site 516 (Rio Grande Rise) have not varied by more than ∼±0.20‰ over the last ∼80 Myr. In contrast, the δ^(44/40)Ca compositions of Mesozoic and Early Cenozoic fossil corals are ∼1‰ lighter than those of modern corals. The observed change in coral δ^(44/40)Ca does not likely reflect secular variations in seawater δ^(44/40)Ca. Instead, we propose that it reflects a vital effect of calcification – specifically, a sensitivity of coral Ca isotope discrimination to changing seawater [Ca] and/or pH. Support for this hypothesis comes from the presence of an empirical correlation between our coral δ^(44/40)Ca record and records of seawater [Ca] and pH since the Mesozoic (Lowenstein et al., 2003 and Hönisch et al., 2012). We explore various mechanisms that could give rise to such a vital effect, including: (1) changes in calcification rate, (2) changes in proton pumping in exchange for Ca^(2+), (3) variable Rayleigh distillation from an isolated calcifying fluid, and (4) changes in the calcium mass balance of the extracellular calcifying fluid (termed here the “leaky Ca model”). We test for the dependence of seawater δ^(44/40)Ca on external seawater [Ca] by measuring the δ^(44/40)Ca of cultured corals grown in seawater solutions with [Ca] ranging from 10 to 15 mmol/kg. Corals grown under elevated [Ca] conditions show a slight, ∼0.15‰ depletion of δ^(44/40)Ca at higher seawater [Ca] – a supportive but not definitive result

A modern scleractinian coral with a two-component calcite–aragonite skeleton

Until now, all of the ca. 1,800 known modern scleractinian coral species were thought to produce skeletons exclusively of aragonite. Asymbiotic Paraconotrochus antarcticus living in the Southern Ocean is the first example of an extant scleractinian that forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. This discovery adds support to the notion that the coral skeletal formation process is strongly biologically controlled. Mitophylogenomic analysis shows that P. antarcticus represents an ancient scleractinian clade, suggesting that skeletal mineralogy/polymorph of a taxon, once established, is a trait conserved throughout the evolution of that clade.One of the most conserved traits in the evolution of biomineralizing organisms is the taxon-specific selection of skeletal minerals. All modern scleractinian corals are thought to produce skeletons exclusively of the calcium-carbonate polymorph aragonite. Despite strong fluctuations in ocean chemistry (notably the Mg/Ca ratio), this feature is believed to be conserved throughout the coral fossil record, spanning more than 240 million years. Only one example, the Cretaceous scleractinian coral Coelosmilia (ca. 70 to 65 Ma), is thought to have produced a calcitic skeleton. Here, we report that the modern asymbiotic scleractinian coral Paraconotrochus antarcticus living in the Southern Ocean forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. P. antarcticus and Cretaceous Coelosmilia skeletons share a unique microstructure indicating a close phylogenetic relationship, consistent with the early divergence of P. antarcticus within the Vacatina (i.e., Robusta) clade, estimated to have occurred in the Mesozoic (ca. 116 Mya). Scleractinian corals thus join the group of marine organisms capable of forming bimineralic structures, which requires a highly controlled biomineralization mechanism; this capability dates back at least 100 My. Due to its relatively prolonged isolation, the Southern Ocean stands out as a repository for extant marine organisms with ancient traits.Mitogenome sequences data have been deposited in GenBank (MT409109). All other study data are included in the article text and supporting information

Fossil corals as archives of secular variations in seawater chemistry

Records of the elemental and isotopic composition of the oceans can help elucidate the geologic controls on seawater chemistry and climate over million-year timescales. This thesis describes the development of a new fossil coral archive that can be used to reconstruct properties of seawater chemistry for the past ~200 My. It also details the application of this archive to investigate changes in seawater Mg/Ca, Sr/Ca, U/Ca, ¿26Mg, and ¿44Ca over the Mesozoic and Cenozoic. Results of diagenetic tests used to validate ~60 fossil coral samples for studies of seawater paleochemistry are presented in Chapters 2 and 4. The validated samples range in age from Triassic through Recent. X-ray diffractometry, scanning electron microscopy, petrographic microscopy, cathodoluminescence microscopy, and micro-raman spectroscopy studies indicate that sample mineralogy is preserved (as aragonite). Studies of 87Sr/86Sr, carbonate clumped isotopes, trace elements sensitive to diagenesis, He/U dating, and U isotopes are used to screen for geochemical signs of alteration. Records of seawater chemistry inferred from validated fossil coral samples are presented in Chapters 2-4. Mg/Caseawater inferred from fossil corals (Chapter 2) is low during the Mesozoic, and increases by a factor of ~5 between 80 Ma and today ¿ compatible with existing reconstructions. The record helps improve our understanding of the timing of Mg/Caseawater changes since the Triassic. Inferred Sr/Caseawater (Chapter 2) varies between 8 and 13 mmol/mol since ~200 Ma, with a maximum in the Late Cretaceous. This result is consistent with reconstructions from benthic foraminifera and fossil fish teeth. A record of ¿26Mgseawater from fossil corals (Chapter 3) helps distinguish between two existing records that give conflicting results, and indicates that the fraction of Mg removed from seawater as dolomite has not changed significantly over the Cenozoic. A reconstruction of fossil coral U/Ca (Chapter 4) suggests that [U]seawater has increased by a factor of ~2 since the Eocene, with implications for our understanding of past seawater [CO32-] and the importance of U removal in reducing sediments. In Chapter 5, a record of ¿44Ca from fossil corals is presented. This record may reflect changes in coral Ca isotope discrimination through time, rather than changes in ¿44Caseawater

A Cenozoic record of seawater Mg isotopes in well-preserved fossil corals

Reconstructions of seawater Mg isotopic composition (δ^(26)Mg) can provide novel insights into the processes that control the major ion chemistry of seawater over geologic time scales. A key period of interest is the Cenozoic (ca. 65 Ma to today), during which the Mg/Ca ratio of seawater increased by a factor of 2–3. However, two published records of seawater δ^(26)Mg over the Cenozoic disagree, making it difficult to draw conclusions about mechanisms driving seawater Mg/Ca change over the past 65 m.y. Here we present a new record of seawater δ^(26)Mg from a set of well-preserved fossil corals, ranging in age from Paleocene to Recent. Fossil coral δ^(26)Mg decreases by ∼0.3‰ between the early Cenozoic and the Oligocene, then increases by ∼0.15‰ between the Oligocene and present, in strong agreement with the published record derived from bulk pelagic carbonate. Together with this existing record, our fossil coral data suggest that the rise in [Mg]_(seawater) over the Cenozoic was mainly driven by an increase in Mg silicate weathering or a decline in Mg uptake in marine silicates. In contrast, we suggest that changes in the rate of carbonate weathering and dolomite formation likely played a minor, but not insignificant, role in the global Mg cycle over the Cenozoic

A Cenozoic record of seawater Mg isotopes in well-preserved fossil corals

Reconstructions of seawater Mg isotopic composition (δ^(26)Mg) can provide novel insights into the processes that control the major ion chemistry of seawater over geologic time scales. A key period of interest is the Cenozoic (ca. 65 Ma to today), during which the Mg/Ca ratio of seawater increased by a factor of 2–3. However, two published records of seawater δ^(26)Mg over the Cenozoic disagree, making it difficult to draw conclusions about mechanisms driving seawater Mg/Ca change over the past 65 m.y. Here we present a new record of seawater δ^(26)Mg from a set of well-preserved fossil corals, ranging in age from Paleocene to Recent. Fossil coral δ^(26)Mg decreases by ∼0.3‰ between the early Cenozoic and the Oligocene, then increases by ∼0.15‰ between the Oligocene and present, in strong agreement with the published record derived from bulk pelagic carbonate. Together with this existing record, our fossil coral data suggest that the rise in [Mg]_(seawater) over the Cenozoic was mainly driven by an increase in Mg silicate weathering or a decline in Mg uptake in marine silicates. In contrast, we suggest that changes in the rate of carbonate weathering and dolomite formation likely played a minor, but not insignificant, role in the global Mg cycle over the Cenozoic

A Cenozoic record of seawater uranium in fossil corals

We measured U/Ca ratios, 4He concentrations, 234U/238U, and 238U/235U in a subset of well-preserved aragonitic scleractinian fossil corals previously described by Gothmann et al. (2015). Comparisons of measured fossil coral He/U ages with the stratigraphic age demonstrate that well-preserved coral aragonite retains most or all of its radiogenic He for 10’s of millions of years. Such samples must be largely or entirely free of alteration, including neomorphism. Measurements of 234U/238U and 238U/235U further help to characterize the fidelity with which the original U concentration has been preserved. Analyses of fossil coral U/Ca show that the seawater U/Ca ratio rose by a factor of 4–5 between the Early Cenozoic and today. Possible explanations for the observed increase include (1) the stabilization of U in seawater due to an increase in seawater [CO32−], and a resulting increase in UO2-CO3 complexation as originally suggested by Broecker (1971); (2) a decrease in the rate of low-temperature hydrothermal alteration from Early Cenozoic to present, leading to a diminished U sink and higher seawater [U]; or (3) a decrease in uranium removal in reducing sediments, again leading to higher seawater [U]

Fossil corals as an archive of secular variations in seawater chemistry since the Mesozoic

The files included here contain supplementary data for the article (http://dx.doi.org/10.1016/j.gca.2015.03.018)