Multidecadal variations in Southern Hemisphere atmospheric ¹⁴C: Evidence against a Southern Ocean sink at the end of the Little Ice Age CO₂ anomaly

Abstract

Northern 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.

The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 cal kBP)

“…Consideration of regional and seasonal growth offsets in tree-ring 14 C. While 14 C offsets between the Southern and Northern Hemispheres are well documented (McCormac et al 1998;Stuiver and Braziunas 1998;Hogg et al 2009;Turney et al 2016a), offsets within the Northern Hemisphere are less well understood. Intrahemispheric offsets were predicted by a global tracer transport model using ocean boundary conditions (Braziunas et al 1995) to be on the order of 8 14 C yr or less in the Northern Hemisphere except at very high latitudes (>70°N).…”

SHCal20 Southern Hemisphere Calibration, 0–55,000 Years cal BP

“…Data sets 3-8 and 3-9 are composed from Wk decadal alpha-cellulose AMS analyses spanning the interval 1705-1945 CE and published in Turney et al (2016). This publication also contained complementary data from Lake Tay (Western Australia) and Campbell Island (NZ).…”

The Influence of Calibration Curve Construction and Composition on the Accuracy and Precision of Radiocarbon Wiggle-Matching of Tree Rings, Illustrated by Southern Hemisphere Atmospheric Data Sets from AD 1500–1950

“…Decadal measurements from Hihitahi Forest Park (set 7), Doughboy Bay, Stewart Island, NZ (set 8), Lake Tay, Western Australia (set 9) and Campbell Island, NZ (set 10) were made by the Waikato AMS laboratory and published in Turney et al (2016). The alpha-cellulose wood fraction was dated with details of the pretreatment methods given in Turney et al (2016). The youngest samples (i.e., AD 1940(i.e., AD -1950 from the resin-rich Doughboy and Lake Tay sites were subject to an initial solvent extraction to remove mobile components (resins, etc.)…”