Moving beyond the age–depth model paradigm in deep-sea palaeoclimate archives: dual radiocarbon and stable isotope analysis on single foraminifera

Bryan C. Lougheed,Lukas Wacker,Brett Metcalfe,Ulysses S. Ninnemann

doi:10.5194/cp-14-515-2018

Abstract

Abstract. Late-glacial palaeoclimate reconstructions from deep-sea sediment archives provide valuable insight into past rapid changes in ocean chemistry. Unfortunately, only a small proportion of the ocean floor with sufficiently high sediment accumulation rate (SAR) is suitable for such reconstructions using the long-standing age–depth model approach. We employ ultra-small radiocarbon (14C) dating on single microscopic foraminifera to demonstrate that the long-standing age–depth model method conceals large age uncertainties caused by post-depositional sediment mixing, meaning that existing studies may underestimate total geochronological error. We find that the age–depth distribution of our 14C-dated single foraminifera is in good agreement with existing bioturbation models only after one takes the possibility of Zoophycos burrowing into account. To overcome the problems associated with the age–depth paradigm, we use the first ever dual 14C and stable isotope (δ18O and δ13C) analysis on single microscopic foraminifera to produce a palaeoclimate time series independent of the age–depth paradigm. This new state of the art essentially decouples single foraminifera from the age–depth paradigm to provide multiple floating, temporal snapshots of ocean chemistry, thus allowing for the successful extraction of temporally accurate palaeoclimate data from low-SAR deep-sea archives. This new method can address large geographical gaps in late-glacial benthic palaeoceanographic reconstructions by opening up vast areas of previously disregarded, low-SAR deep-sea archives to research, which will lead to an improved understanding of the global interaction between oceans and climate.

Highlights

The past seven decades in palaeoceanography research have produced a wealth of valuable palaeoclimate data from the calcareous, foraminiferal ooze contained in deep-sea sediment archives, greatly improving our understanding of past ocean chemistry and palaeoclimate (Bond et al, 1993; Emiliani, 1955; Epstein et al, 1951; Shackleton, 1967; Urey, 1947)
We employ ultra-small radiocarbon (14C) dating on single microscopic foraminifera to demonstrate that the long-standing age–depth model method conceals large age uncertainties caused by post-depositional sediment mixing, meaning that existing studies may underestimate total geochronological error
We find that the age–depth distribution of our 14C-dated single foraminifera is in good agreement with existing bioturbation models only after one takes the possibility of Zoophycos burrowing into account

Summary

Introduction

The past seven decades in palaeoceanography research have produced a wealth of valuable palaeoclimate data from the calcareous, foraminiferal ooze contained in deep-sea sediment archives, greatly improving our understanding of past ocean chemistry and palaeoclimate (Bond et al, 1993; Emiliani, 1955; Epstein et al, 1951; Shackleton, 1967; Urey, 1947). The long-standing geochronological method that has been applied to these sediment archives since the inception of palaeoceanography as a field of study, the age–depth model method, relies on the geological law of superposition. This law states that sediment age increases progressively with sediment core depth. The age–depth model method, as applied to deep-sea sediment cores, involves first slicing sediment cores into discrete core depth intervals of 1 cm thickness or greater. Blaauw and Christen, 2011; Bronk Ramsey, 2008) are used to interpolate ages for all discrete depth intervals of the sediment core Statistical methods (e.g. Blaauw and Christen, 2011; Bronk Ramsey, 2008) are used to interpolate ages for all discrete depth intervals of the sediment core

Methods

Results

Conclusion