Last Glacial Maximum and deglacial abyssal seawater oxygen isotopic ratios

Carl Wunsch

doi:10.5194/cp-12-1281-2016

Abstract

Abstract. An earlier analysis of pore-water salinity (chlorinity) in two deep-sea cores, using terminal constraint methods of control theory, concluded that although a salinity amplification in the abyss was possible during the LGM, it was not required by the data. Here the same methodology is applied to δ18Ow in the upper 100 m of four deep-sea cores. An ice volume amplification to the isotopic ratio is, again, consistent with the data but not required by it. In particular, results are very sensitive, with conventional diffusion values, to the assumed initial conditions at −100 ky and a long list of noise (uncertainty) assumptions. If the calcite values of δ18O are fully reliable, then published enriched values of the ratio in seawater are necessary to preclude sub-freezing temperatures, but the seawater δ18O in pore fluids does not independently require the conclusion.

Highlights

Based upon the work of McDuff (1985), Schrag and DePaolo (1993), Schrag et al (1996, 2002), Adkins and Schrag (2001, 2003), Insua et al (2014), and several others, on the properties of pore waters in abyssal cores, inferences have been made about the salinity and temperature of the regional and global abyssal oceans during the Last Glacial Maximum (LGM)
A summary of the central conclusion (e.g. Adkins et al, 2002) would be that the deep ocean was almost everywhere near freezing temperatures, with an abyssal salinity much above the global volume average, in the Southern Ocean. Those inferences have become a part of the ongoing discussion of climate physics, including the whereabouts of global carbon during the LGM, and are a standard against which models are being tested: e.g. OttoBliesner et al (2006), Intergovernmental Panel on Climate Change (2013), and Kobayashi et al (2015)
Miller (2014) and Miller et al (2015) have challenged this interpretation showing, using a Monte Carlo method, that the uncertainties of the inferences were too great to assert that the LGM abyssal stratification could be determined with useful accuracy

Summary

Introduction

Based upon the work of McDuff (1985), Schrag and DePaolo (1993), Schrag et al (1996, 2002), Adkins and Schrag (2001, 2003), Insua et al (2014), and several others, on the properties of pore waters in abyssal cores, inferences have been made about the salinity and temperature of the regional and global abyssal oceans during the Last Glacial Maximum (LGM). Adkins et al, 2002) would be that the deep ocean was almost everywhere near freezing temperatures, with an abyssal salinity much above the global volume average, in the Southern Ocean. Those inferences have become a part of the ongoing discussion of climate physics, including the whereabouts of global carbon during the LGM, and are a standard against which models are being tested: e.g. OttoBliesner et al (2006), Intergovernmental Panel on Climate Change (2013), and Kobayashi et al (2015). Miller (2014) and Miller et al (2015) have challenged this interpretation showing, using a Monte Carlo method, that the uncertainties of the inferences were too great to assert that the LGM abyssal stratification could be determined with useful accuracy

Objectives

Discussion

Conclusion