Deep biosphere and element cycling in sediments

Climate model predictions and observations reveal a future trend towards increasing ocean deoxygenation and the expansion of oxygen minimum zones (OMZ) impinging on continental shelf areas. Similar episodes of widespread ocean anoxia, so called oceanic anoxic events (OAEs) are associated with many biotic crises in Earth’s history. In order to predict the Earth system's response to changes in greenhouse gas concentrations and radiative forcing, a sound understanding of OMZ-type biogeochemical cycling in open ocean shelf settings is required.

The Benguela Upwelling System (BUS) off southwest Africa is an excellent site to study these biogeochemical processes re­gard­less of whether they are mi­cro­bi­ally cata­lyzed or in­or­ganic. However, there is sparse know­ledge on the low-fre­quency vari­ab­il­ity of the geochem­ical (redox) con­di­tions on longer (i.e. > decadal) time scales and how the re­spons­ible changes in phys­ical driv­ing factors are related to climate changes.

Therefore, my PhD work focuses on studying vari­ations in bottom water oxygen concentration and the related productivity signal of inner-shelf diatomaceous mud belt deposits during Holo­cene and Latest Pleis­to­cene. Sediment cores from two cross-shelf transects at 23°S and 25°S (75 – 250m water depth), were sampled during RV Meteor expedition M157 and have been 14C-AMS dated with ages going back to 46 kyrs.

To answer the question of whether oxy­gen is avail­able, lim­ited or ab­sent and for how long, redox-sensitive elements (e.g. Fe, Mn, Mo, U) will be used as proxies for vari­ations of pre­vail­ing geo­chem­ical conditions.

To study the temporal and spatial variability of organic matter degradation and its preservation potential, sedimentary organic carbon and nitrogen data including δ¹³C and δ¹⁵N will be interpreted together with biogeochemical reactive transport modelling of early diagenetic processes.

Hypothesis

The intensity and expansion of anoxia is documented in high-resolution sediment archives.

Contacts: Michael Kossack, Matthias Zabel