Southern Ocean warming preceded rising CO2 during the last deglaciation

Evidence from marine sediment cores indicates that Southern Ocean warming pre-dated the rise of atmospheric CO2 during the last deglaciation. Radiocarbon dating of micro organisms living on the deep ocean floor and in surface waters in a marine core collected in the western tropical Pacific was used to determine the relative chronology of warming in the Southern Ocean near Antarctica and rising CO2 during the last deglaciation. The results provide evidence that that the Southern Ocean off Antarctica warmed by ~2°C between 19,000 and 17,000 years before the present, about 1,000 years before the rise in atmospheric CO2.

Introduction

Research has determined that the Southern Ocean warmed by about  2°C early in the last deglaciation.   The timing of this warming relative to tropical surface water warming and increasing atmospheric CO2 concentration has remained uncertain. This study establishes a common chronology to enable the comparison of the timing of high latitude Southern Ocean warming with rising atmospheric CO2 concentration and tropical ocean surface warming during the last deglaciation.

Deep water is formed when warm water becomes more saline and sinks. Deep water formation occurs at high latitudes in the north and south Atlantic and in southern high latitudes in the Pacific. As deep water moves with ocean currents its salinity and temperature are nearly invariant and can therefore be used as a signature that identifies the source of the water. The Pacific is the best candidate for testing the evolution of warming in the southern ocean because by volume it is the largest deep water reservoir.

Observations

In this study shells from bottom-dwelling micro-organisms and those living in surface waters (plankton) have been studied in marine sediments corresponding to the beginning of the last deglaciation, about 19,000 to 17,000 years ago. Temperature proxies, Delta-18oxygen and magnesium/calcium (Mg/Ca) ratios from these shells, have been used to determine the temperature of the waters in which the micro-organisms lived. The marine sediment core was taken from the Morotai Basin near North Maluku, Indonesia, very near the equator. At this location the record from the species of micro-organisms dwelling on the bottom is a proxy for properties of Southern Hemisphere deep water formed in high latitudes near Antarctica. The record from the species of surface-dwelling micro-organisms (plankton) is a proxy for tropical Pacific surface waters. Together the bottom and surface records make it possible to determine the relative timing of high-latitude (near Antarctica) versus low-latitude (tropical) changes in the southern ocean during the last deglaciation. Unlike previous studies this study used radiocarbon (carbon-14) dating to establish the chronology of the sediment layers which enables a comparison with atmospheric CO2 concentration derived from other sources.

The marine sediment core contains shell sediments that accumulated at rates of 50 to 80 cm per thousand years over the last deglaciation. This core was recovered from a depth of 2114 meters, where the bottom-dwelling micro-organisms reflect the properties of the upper Pacific Deep Water, which was formed at high latitudes near Antarctica. The core site is located near the equator, where the delta-18oxygen and Mg/Ca of the surface micro-organisms record the temperature and salinity of tropical Pacific surface water. Sampling this core at a centimeter scale provides a resolution for each sample of about 25 to 50 years.

Analysis

Comparison of Southern Ocean deep and surface water temperatures with the EPICA Dome C CO2 record. The averaged mean longitude spring insolation (21 August to 20 November) at 65°S and the Dome Fuji ice core temperature deviation (deltaT-site) relative to the mean of the last 10 thousand years is shown. The yellow shading spans the time period between the initial warming of deep-sea temperatures and the onset of tropical Pacific sea surface warming. The deep Pacific delta-18O record is based on several species of deep sea plankton blue circles, red circles, black squares, green diamonds, and red squares. The gray shading indicates a 200-year uncertainty in the deep sea ages.

The results indicate that high-latitude Southern Ocean warming occurred before 17,500 years ago and therefore before both the onset of warming of the tropical Pacific surface waters and the increase in atmospheric CO2 concentration. It is estimated that the high latitude ocean warming occurred about a thousand years before the onset of rising atmospheric CO2 and warming tropical surface waters.

Support for this chronology is found in other research that has reported a comparable warming in surface-water records from the mid-latitudes in the south Pacific. At these latitudes the surface waters also warmed by 2°C between 19,000 and 17,000 years ago. The onset and evolution of deglacial warming over the Antarctic continent also matches the record of warming in the high latitude southern ocean. There is evidence that high-elevation glaciers in the mid–southern-latitudes retreated at the same time. This evidence confirms that the onset of deglacial warming throughout the Southern Hemisphere occurred long before deglacial warming began in the tropical surface ocean.

An early warming in the Antarctic means that the mechanism responsible for initiating the deglaciation did not begin in the tropics nor was it initated by CO2 forcing. Both CO2 concentration and the tropical sea surface temperatures (SSTs) did not begin to change until after 18,000 years ago, approximately 1000 years after the deep water delta-18oxygen record indicated that the southern ocean was warming.

The authors suggest a mechanism for how deglaciation was initiated and proceeded.  The trigger for the initial deglacial warming around Antarctica was the change in solar insolation over the southern ocean during the Southern Hemisphere spring. This initiated the retreat of Antarctic sea ice which would have reduced albedo and increased coupling between wind and surface water in the southern ocean. The decreased stratification of the ocean resulted in greater upwelling which would have promoted ventilation of CO2 from the deep sea and which may have been responsible for the subsequent rise in atmospheric CO2.

Southern Hemisphere and Deep-Sea Warming Led Deglacial Atmospheric CO2 Rise and Tropical Warming, Lowell Stott, Axel Timmermann, Robert Thunell, Science 19 Oct 2007:Vol. 318, Issue 5849, pp. 435-438 DOI: 10.1126/science.1143791