Magnetic unmixing of conventional and giant coastal marine magnetofossils during the Paleocene-Eocene Thermal Maximum

Abstract:

Conventional magnetofossils represent magnetosome morphologies biomineralized by known modern magnetotactic bacteria. Giant magnetofossils represent morphologies with no known modern analogue. Both conventional and giant magnetofossil assemblages may record paleoenvironmental information including changes in magnetotactic bacteria diversity and oceanic deoxygenation caused by global warming. The links between conventional and giant magnetofossils with microbial diversity, nutrient cycling, and marine oxygen make them useful for paleoenvironmental applications. Here, we investigate marine Paleocene-Eocene Thermal Maximum (PETM, ~56 m.y.a.) sediments from Wilson Lake, New Jersey. These neritic sediments contain abundant conventional and giant magnetofossils with high morphological disparity, which we document using transmission electron microscopy of magnetic extracts. We test whether principal component analysis of 63 first-order reversal curve (FORC-PCA) datasets can be used to distinguish between these magnetofossil assemblages. Our results suggest that FORC-PCA is sensitive to the presence of giant needles and that this component can be used to identify sediments containing giant magnetofossils. We use this high-resolution dataset, in combination with the ecology of conventional and giant magnetofossils, to provide a more detailed understanding of how this coastal ecosystem responded to rapid planetary warming during the PETM. We hypothesize that increased weathering of iron-rich laterites, water temperature, and air temperature resulted in excess bioavailable iron and caused the expansion of the oxic-anoxic interface (OAI) at the onset of the PETM. In our model, the remainder of the PETM may have been punctuated by intervals of more seasonally derived organic matter and a smaller OAI, a product of the baseline increase in seasonality during the PETM. We show that FORC-PCA is a robust, cost-effective, non-destructive technique that may be used to track the relative timing and extent of oceanic deoxygenation stimulated by rapid planetary change.