Biominearlization in Deep-sea Corals and the Causes of 'Vital Effects' More Generally

Abstract

The stable isotope compositions of biogenic carbonates have been used for paleoceanographic and paleoclimatic reconstructions for decades, and produced some of the most iconic records in the field. However, we still lack a fully mechanistic understanding of the stable isotope proxies, especially the biological overprint on the environmental signals termed “vital effects”. A ubiquitous feature of stable isotope vital effects in marine calcifying organisms is a strong correlation between δ18O and δ13C in a range of values that are depleted from inorganic calcite/aragonite. Two mechanisms have been proposed to explain this correlation, one based on kinetic isotope effects during CO2(aq)-HCO3- inter-conversion, the other based on equilibrium isotope exchange during pH dependent speciation of the dissolved inorganic carbon (DIC) pool. Neither mechanism explains all the stable isotope features observed in biogenic carbonates. Here we present a fully kinetic model of biomineralization and its isotope effects using deep-sea corals as a test organism. A key component of our model is the consideration of the enzyme carbonic anhydrase in catalyzing the CO2(aq)-HCO3- inter-conversion reactions in the extracellular calcifying fluid (ECF). We find that the amount of carbonic anhydrase not only modulates the carbonate chemistry of the calcifying fluid, but also helps explain the slope of the δ18O-δ13C correlation. Differences in CA activity in the biomineralization process can possibly explain the observed range of δ18O-δ13C slopes in different calcifying organisms. A mechanistic understanding of stable isotope vital effects with numerical models can help us develop better paleoceanographic tracers. We will show preliminary evidence for how this model can improve Me/Ca proxies (eg: Mg/Ca and B/Ca) as well.

Speaker Bio

Jess Adkins is the Smits Family Professor of Geochemistry and Global Environmental Science in the Department of Geological and Planetary Sciences at Caltech. He is a member of the Linde Center for Global Environmental Sciences and on the faculty of the Department of Engineering and Applied Science. Jess started at Caltech in 2000 after receiving a BS in Chemistry from Haverford College and a PhD in Chemical Oceanography from the MIT/WHOI Joint Program in 1998. At MIT Jess worked with Ed Boyle on developing deep-sea corals as a new archive of paleoclimate. They coupled U-series and radiocarbon dating as way to measure the past ventilation rate of the deep ocean and used stable isotope and trace metal records to document rapid climate change in the deep-sea. This work continues in the Adkins lab to this day. Between graduate school and Caltech, Jess was a Lamont Post-doc Fellow at LDEO and a NOAA Global Change Post-doc Fellow at the Universities of Minnesota and California, Irvine with Larry Edwards and Ellen Druffel. Early in his career Jess collaborated with Dan Schrag to measure the T/S plot for the deep ocean at the Last Glacial Maximum. This work has continued in the Adkins lab as an investigation of the density structure of the past water column and how novel observations fit with theories of the overturning circulation to explain the ocean’s role in glacial-interglacial climate change. Jess has worked on stalagmite records of tropical climate change, biomineralization mechanisms in corals, and iron isotopes in the modern ocean. Recently, together with Alex Sessions, Jess has lowered the detection limit for sulfur isotopes and developed a method for compound specific δ34S in organic matter using a MC-ICP-MS coupled to a GC. This has led to work on the sulfur cycle more generally, investigating the modern budget of river sulfur isotopes, ice core records of volcanic eruptions, and measuring the history of marine sulfate isotopic change from the Archean to the Cenozoic. In an ongoing collaboration with Will Berelson at USC, the Adkins group has measured the dissolution rate of calcite in seawater with a new techniques that is very precise even close to equilibrium. We have found that the enzyme carbonic anhydrase catalyzes the reaction between CO2 and CaCO3, thus providing new constraints on the dual effects of solution chemistry and surface energy on carbonate dissolution in the lab and in the ocean. Jess has used multiple deep submergence tools to explore the bottom of the ocean and has spent over 150 days at sea on expeditions to sample fossil deep-sea corals, take long sediment cores, and measure the in situ rate of carbonate dissolution in the water column.

Jess was awarded the Houtermans medal from the European Association of Geochemistry in 2003. He received the Ruth and Paul Fye MIT/WHOI and the Geochemical Society Best Paper Awards in 2005 and 2009 respectively. He was the Chow Lecturer at The Scripps Institute of Oceanography in 2017. Adkins was a Visiting Scientist at the LSCE in Gif-sur-Yvette, France in 2007.