Mercury Stable Isotopes as a Proxy of Photic Zone Euxinia

Arizona State University, Scottsdale AZ

Investigators

Abstract

The goal of this project is to develop a new approach to identify periods in Earth history when hydrogen sulfide-rich waters reached the shallow depths of the oceans which sunlight easily penetrates, known as the "photic zone." This is the part of the ocean in which biology is most vigorous. In particular, the photic zone is where about 25% of photosynthesis happens on Earth each year. When hydrogen sulfide-rich waters enter the photic zone, they devastate animal life and may also hinder photosynthesis, causing massive disruption to ocean ecosystems. Many mass extinctions in Earth's past may have been associated with hydrogen sulfide in the photic zone, but the details of how ocean chemistry and ecosystems are linked are poorly understood because there are few good ways to detect shallow water sulfide in the geologic record. This research is therefore of fundamental importance to understanding the course of life through Earth's history. However, the research is also relevant to predicting our future because the occurrence of hydrogen sulfide in shallow waters is now more likely due to human activities. These include excess use of fertilizers and global warming. This research therefore may help to shed light on the sorts of ecosystem changes that might be expected as the human impact on ocean chemistry accelerates. As part of the work this team will also develop a novel digital education resource to teach students about changes in ocean chemistry through time. Specifically, investigators will explore the use of mercury (Hg) stable isotopes as a proxy for photic zone euxinia (PZE) in Earth's ancient oceans. PZE occurs when anoxic, sulfide-rich waters expand into the photic zone. Although this condition is rare today, PZE is thought to have been widespread in some ancient oceans with potentially profound effects on marine and terrestrial ecosystems. However, developing convincing evidence of PZE in the past has been difficult. In this proposal, investigators hypothesize that Hg stable isotope compositions in marine sedimentary rocks may serve as a new proxy of PZE. The basis of this novel proxy is that Hg undergoes unique mass independent isotope fractionation (MIF) during aqueous photoreduction of Hg(II) when complexed by reduced sulfur ligands. Specifically, photoreduction of Hg(II) bound to oxygen-containing ligands (e.g., carboxyl) leads to positive MIF (enrichment of odd-mass-number isotopes, 199Hg and 201Hg in the residual Hg(II)). In contrast, photoreduction of Hg(II) bound to reduced sulfur ligands (e.g., thiol) produces negative MIF. Consistent with this rationale, a pilot study on Hg isotope compositions in Mesoproterozoic shales showed zero to slightly positive MIF in sediments deposited under oxic conditions and distinct negative MIF under sulfidic conditions, thus supporting the hypothesis. Researchers will further validate this emerging new proxy by: 1) investigating Hg biogeochemical cycles and isotope fractionation in a modern shallow-water sulfide-rich environment, which serves as a modern analog of PZE; and 2) measuring Hg isotope compositions in ancient sedimentary rocks that have documented evidence of PZE. This research will be complemented by the first experimental studies on Hg photochemistry in sulfidic water and the corresponding isotope fractionation, which will be carried out through a collaboration. The researchers will also examine the new rhenium (Re) isotope redox proxy in the same suites of rock samples that will be analyzed for Hg isotopes, which might benefit the interpretation of both isotope systems. Re isotopes are a very novel paleoredox proxy that can potentially distinguish between suboxic and anoxic/euxinic conditions. Thus, the coupled study of Hg and Re isotopes should provide a more complete picture of the redox structure of the ancient ocean, providing better constraints on both proxies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.