Hydrodynamic Influences on Proxy Records in Marine Sediment Drifts: Challenges, Opportunities and Implications for Paleoclimate Reconstruction

Marine drift deposits provide amongst the thickest and most continuous paleoclimate archives. Exceptionally well-dated sediment cores from these settings are used to constrain leads, lags and assess synchronicity in climate signals through multiproxy approaches, assuming that proxy signals in co-deposited sedimentary components are coeval with respect to their time of formation.

Here, we report substantial ¹⁴C-age discrepancies among co-deposited foraminifera, organic carbon (OC), alkenones, and terrestrial leaf waxes, from a sediment core at the Shackleton Sites on the SW Iberian margin. Such discrepancies are attributed to lateral transport and subsequent deposition of 'pre-aged' organic matter (OM), implying possible spatiotemporal offsets among derived proxy signals at the study site. A model is used to account for a distribution of OM ages under the assumption that older observed ages result from the combination of both autochthonous (co-eval) and allochthonous (pre-aged) material, indicating a minor to moderate contribution (15-20%) of substantially pre-aged (18,500-49,900 years) material. This allows to deconvolute the alkenone-derived temperature record, which suggests that the addition of older, colder, allochthonous alkenones leads to a smoothing of the record during the last interglacial. In order to further assess the influence of hydrodynamic sorting processes on sedimentary OM signatures and ages, we performed grain size-specific OC ¹⁴C analyses on surface sediments from a range of depositional settings. A strong and ubiquitous ¹⁴C age-grain-size dependence highlights the pervasive impact of hydrodynamic sorting on bulk sediment OC age and content, most strongly manifested in finer grain-size fractions (2-10 μm). Similar effects were also observed downcore at the Shackleton sites, where the magnitude of the ¹⁴C age offsets among grain-size fractions co-varied with paleochanges in the ocean current strength that, in turn, are related to abrupt climate events of the last deglaciation.

This work highlights the importance of developing comprehensive and critical assessments of the impact of hydrodynamic sorting processes on marine drift deposits prior to embarking on detailed paleoclimate investigations using material from these benchmark depositional settings.