Prof Christopher Ramsey

Abstract:

AbstractNorthern Hemisphere‐wide cooling during the Little Ice Age (LIA; 1650–1775 Common Era, C.E.) was associated with a ~5 ppmv decrease in atmospheric carbon dioxide. Changes in terrestrial and ocean carbon reservoirs have been postulated as possible drivers of this relatively large shift in atmospheric CO2, potentially providing insights into the mechanisms and sensitivity of the global carbon cycle. Here we report decadally resolved radiocarbon (14C) levels in a network of tree‐ring series spanning 1700–1950 C.E. located along the northern boundary of, and within, the Southern Ocean. We observe regional dilutions in atmospheric radiocarbon (relative to the Northern Hemisphere) associated with upwelling of 14CO2‐depleted abyssal waters. We find the interhemispheric 14C offset approaches zero during increasing global atmospheric CO2 at the end of the LIA, with reduced ventilation in the Southern Ocean and a Northern Hemisphere source of old carbon (most probably originating from deep Arctic peat layers). The coincidence of the atmospheric CO2 increase and reduction in the interhemispheric 14C offset imply a common climate control. Possible mechanisms of synchronous change in the high latitudes of both hemispheres are discussed.

Abstract:

The research project 'Response of Humans to Abrupt Environmental Transitions' (RESET) used tephra layers to tie together and synchronise the chronologies of stratigraphic records at archaeological and environmental sites. With the increasing importance of tephra as chronological markers in sedimentary sequences, both in this project and more generally, comes a requirement to have good estimates for the absolute age of these volcanic horizons. This paper summarises the chronology of the key tephra in the RESET tephra lattice in the time range 10-60 ka BP, from the existing literature, from papers produced as part of the RESET project, and reanalysis conducted for this paper. The paper outlines the chronological approach taken to the dating of tephra within the RESET project, and the basis for further work, as part of the INTIMATE (INTegrating Ice core MArine and TErrestrial records) initiative. For each of the tephra layers in the lattice, the existing literature is discussed and, where relevant date estimates updated using the latest radiocarbon calibration curves (IntCal13 and Marine13) and methods. Maps show the approximate extent of tephra finds, giving a visual indication of the coverage of the lattice in different time-periods.