Julio Sepúlveda

Organic and stable isotope biogeochemistry, geobiology, biomarker research, paleoceanography, paleoclimate, extreme climates, mass extinction events

My research group studies the interplay between microorganisms, biogeochemical processes, and climate in contemporary settings (e.g., continental margins, marine oxygen minimum zones, extreme environments), and paleo-ecosystems across major climatic/biotic transitions in Earth history (e.g., mass extinction events, greenhouse climates, glacial-interglacial transitions). We approach these systems with a focus on the structures, distributions, and stable isotope composition of cell membrane lipids (biomarkers) that can be identified in water bodies, sediments, soils, and their fossilized remains preserved in the rock record.

Extreme climates over the Phanerozoic

Periods of extreme climates in the geological record are associated with changes in ocean geochemistry and varying degrees of extinction, and can serve as partial analogues to study ecosystem resilience to projected scenarios of ocean deoxygenation and acidification. However, our current understanding of ecosystem’s resilience derives almost exclusively from the body fossil record, thus disregarding important microbial biota lacking hard, fossilizable skeletons, which exert strong control on ocean geochemistry and climate.

My current efforts aim to unravel the role of photoautotrophic eukaryotes and prokaryotes, and chemosynthetic prokaryotes in sustaining carbon fixation across periods of extreme climate change and extinction, as well as to unravel the environmental responses (e.g., water column stratification, oxygenation, nutrient budgets) to CO₂ forcing. I currently study five major climatic events associated with varying degrees of extinction during the Mesozoic and Cenozoic: (a) end–Triassic mass extinction; (b) end-Cretaceous mass extinction; (c) early Jurassic OAE; (d) mid-late Cretaceous OAEs; (e) Paleocene-Eocene Thermal Maximum (PETM).

Biogeochemistry of oxygen minimum zones: Past, present, and future

Marine oxygen minimum zones (OMZs) are key areas of the world’s ocean for the recycling of carbon, nutrients and the release of greenhouse gases through microbial processes, therefore influencing the global climate system.  OMZs have expanded over the last five decades, and this expansion is expected to continue, in parallel with ocean acidification, in response to current and projected trends of climate change. Therefore, reliable proxies for oxygenation and microbial processes in the past are required to constrain the response of OMZs to future warming scenarios.

My current research efforts aim to illuminate microbially–driven processes in modern oxygen–depleted environments through the tandem analysis of lipid biomarkers and gene-based techniques, as well as the validation of biomarkers as proxies for paleoxygenation and ocean biogeochemistry in sedimentary records.  I presently study the provenance and diversity of microbial biomarkers across five distinctive oxygen-depleted marine environments. Results from this study are expected to improve our understanding of biomarkers for microbial metabolism involved in nitrogen and sulfur cycling under different degrees of anoxia/euxinia.  Also, I currently study the biogeochemical evolution of the Chilean OMZ over past warm climatic transitions (i.e., Marine Isotope Stages 5 and 11, mid-Holocene warm period). A particular interest is to elucidate the relative role of different denitrifying processes (denitrification vs. anaerobic ammonium oxidation) on controlling nutrient budgets and climate variability. These results should greatly facilitate our interpretation of present and past responses of OMZs to natural and anthropogenic forcing, thus assisting our understanding of potential future scenarios.

Carbon cycling along continental margins and paleoclimatology

Continental margins are important components of the global carbon cycle as they support high levels of primary productivity and increased carbon burial rates.  I am interested in the cycling of marine and terrestrial organic mater along continental margins subject to increasingly human pressure. My work also focuses on the validation of organic geochemical proxies for paleoclimate reconstructions.  Current areas of study include the fjord system of Patagonia and the upwelling area of central and northern Chile.