Our current research is focused on two main topics, (i) biogeochemical element cycles under different redox settings and (ii) microbially mediated and inorganic geochemical processes in the Deep Biosphere.
Biogeochemical element cycling and fluxes in Arctic marine sediments
Reverse and Submarine Weathering in Glacially Influenced Arctic Fjords
(NSF-funded; with R. Aller; student: J. Dotzler; collaborator: G. Steinhoefel, AWI Bremerhaven)
Over long time scales, the inputs of major and minor elements to the ocean by rivers and hydrothermal vents have to be balanced by removal mechanisms of these elements in the marine realm in order for ocean chemistry to remain relatively constant. A long-debated process that sequesters elements in ocean seafloor sediments and affects seawater pH and alkalinity is reverse weathering, which involves the transformation of biogenic silica, such as diatom frustules, to new silicate (clay) minerals. Many aspects of this process still remain unknown, such as reaction rates and products, and global distribution. Previous studies of this process have focused mainly on tropical and subtropical coastal ocean systems. Coastal polar regions, including glacially influenced fjords, likely represent another hotspot of reverse weathering because they receive high inputs of key reactant “ingredients”: biogenic silica and reactive iron and aluminum oxide minerals. At the same time, climate change drives the retreat of Arctic glaciers and is expected to strongly modify sediment delivery to and carbon cycling in these fjords. In this project we aim to fill key gaps in our knowledge of the pathways and products of the reactions involving silicate minerals in sediments and the impacts of these processes on the removal of dissolved ions from seawater in glacially influenced fjords.
The benthic cyc
ling of bioessential trace metals in rapidly changing Arctic fjords
Glacially influenced Arctic fjords are one of the main settings that we are studying with the goal of deciphering biogeochemical element cycles. These boundary systems between the glacial and proglacial zones and the open ocean are strongly affected by the delivery of weathering products from the surrounding glaciers. The key objective for this research is to investigate the sources, transport, and diagenetic redox cycles of glacially derived trace metals across high latitude fjords and the delivery of these metals to high-latitude oceans over geological time scales. In light of the sensitivity of the Arctic to recent climate change, it is critically important to understand the biogeochemical processes and feedback mechanisms that drive primary productivity in these environments.
Biogeochemical element cycling and fluxes in sandy bioturbated sediments
Iron cycling in bioturbated sediments – Fluxes, diagenetic redistribution, and isotopic signatures
(with R. Aller and N. Volkenborn)
This project will fill key gaps in our knowledge of seabed sources, and emphasizes the interconnected effects of bioturbation by infaunal organisms, bottom water O2 concentrations, sedimentary organic matter content, and sediment permeability on Fe cycling and isotopic redistribution in continental margin sediments. Specifically, we will gain a mechanistic understanding of the impact of variable O2 concentrations within infaunal burrows (as a function of irrigation activity and sedimentary setting) and in the overlying water (in response to large scale environmental change) on dissolved Fe fluxes and re-precipitation, the isotopic fractionation related to these processes, and the consequences for isotopic signatures of dissolved Fe in the water column and particulate Fe preserved in sediments.
The Deep Biosphere
Research on the d
eep biosphere in our group focuses on three interlinked key aspects. First, we examine the imprint of fluctuating external paleo-environmental factors on the evolution of biogeochemical processes in the sediment. This approach includes a) the identification of the microbially mediated and secondary inorganic reactions that occur at present and have taken place in the past, and b) the evaluation of transient geochemical signals that give insight into changes in the rates and pathways of these processes over time. Factors that lead to such changes include variability of sediment accumulation rates, variation in organic carbon deposition linked to changes in surface water productivity, and the fluctuating deposition of reactive mineral phases, e.g., Fe-oxides, due to variations in the oceanographic regime. Ultimately, such changes in the geological, oceanographic, and paleoenvironmental settings shape the deep biosphere. The second aspect of our deep biosphere research focuses on the subseafloor carbon cycle. Marine sediments represent an important player in the global carbon cycle. Nonetheless, key aspects of the functioning of this habitat with respect to organic carbon storage, transformations of carbon between different inorganic and organic carbon pools, and the release of dissolved inorganic carbon (DIC) from deep subseafloor sediments to the water column, remain poorly understood. We are particularly interested in diagenetic carbonate formation and the potential link between methanogenesis and silicate weathering.