Particle-hosted sulfur cycling in marine
O -deficient zones
Sinking marine particles are complex biochemical reactors, where microorganisms respire most of the organic matter that was fixed by photosynthesis in the surface ocean. Understanding the controls on organic matter lability, or accessibility to microorganisms, in these particles is a central problem for modeling carbon fluxes and nutrient cycling in the ocean. A hot topic of current debate concerns the role of sulfur cycling in modern O2-deficient zones, where free dissolved sulfide is rarely detected, but the enzymes for microbial sulfate reduction are present. We have been collecting sinking particles using incubators designed at the University of Washington, and then using a combination of radioactive and stable isotope labels to measure reaction rates and characterize solid-phase products.
left: Eastern Tropical North Pacific, April 2018 (image: R. Keil)
Organic matter sulfurization and enhanced carbon burial
Ocean Anoxic Event 2 is the quintessential example of enhanced organic matter burial on a global scale. Thick, organic-rich deposits (right) were the main sinks for the abundant volcanic CO2 that wreaked havoc with the climate and biogeochemical cycles at this time, ~94 million years ago. A key mechanism for enhanced organic matter preservation under anoxic conditions is 'sulfurization,' which makes organic matter inaccessible to microorganisms. To assess the extent and character of sulfurization during this critical period and estimate its impact on the carbon and sulfur cycles, we are investigating rocks that represent this event from all over the globe. We have been finding globally consistent relationships between the intensity of sulfurization and total organic carbon burial across diverse environments. The lessons from this work are applicable to other less extreme perturbations in Earth history, including Pleistocene glacial-interglacial climate changes.
Sulfur cycling in mangrove and wetland ecosystems
Despite their relatively small areal extent, coastal ecosystems have an outsized impact on global carbon burial. They are frequently 'hotspots' for carbon burial, but they can also release CO2 rapidly if disturbed. In the tidally exposed environment at left (Turks and Caicos Islands), thick microbial mats fix and respire carbon via tightly interwoven metabolic strategies, but net carbon preservation is low. By comparing the rates and mechanisms of reactions in this system with those at sites with much higher carbon preservation (e.g., Belize), we can track how different environmental conditions translate into different amounts of local organic carbon burial in these critical environments.
Rates and mechanisms of organic matter transformations
Laboratory experiments allow us to test the hypotheses we develop in the environment and measure important parameters that can be hard to constrain in complex natural systems. For example, under what conditions can organic moieties out-compete iron for available sulfide? How accessible is organic matter to microorganisms after polysulfide exposure? And, does sedimentary sulfur cycling impact the preservation of terrestrial and marine organic matter differently?
The samples at right are incubations of freshwater wetland sediments from Florida and Missouri with different concentrations and distributions of reactive sulfur species, which are isotopically labeled so that we can easily identify reaction products.
Sulfur-isotope records of the global
and local environment
The S isotope compositions of pyrites (FeS2) have been invaluable records of biogeochemistry in the geologic record, but the kinetics of pyritization are complex and depend strongly on the availability and mineralogy of iron as well as sulfide. As a second major sink for sulfide in anoxic sediments, organic matter (OM) provides essential context for reconstructing the distribution and isotopic composition of environmental sulfide. To first order, roughly parallel d34S profiles reflect changes in sulfide, while independent patterns require alternative explanations. We have been applying this framework to a variety of recent and ancient environments, using OM S:C ratios and paired S-isotope records for pyrite and OM to reconstruct both local redox structure and global mechanisms impacting the sulfur and carbon cycles.