Assessment of Variability in Southern Ocean

An assessment of variability in Southern Ocean ¹⁴C reservoir ages from paired ¹⁴C and U/Th dating of solitary corals

Principal Investigators: Brenda Hall, Gideon Henderson (Oxford),
Carlo Baroni (Pisa), and Tom Kellogg

Partially supported by the NOSAMS Laboratory at Woods Hole Oceanographic Institution

The ¹⁴C/¹²C ratio in surface waters of the Southern Ocean is significantly lower than that in the atmosphere due to upwelling of old deep water. This "reservoir age" probably varies in both space and time due to changes in the position and magnitude of such upwelling. Assessment of the recent spatial pattern of natural reservoir ages is made difficult, however, by the presence of significant bomb ¹⁴C. Constraining past temporal change is harder still.

Assessment of the pre-bomb and Holocene history of Southern Ocean reservoir ages is an important goal for three principal reasons:

1. Raised beaches around the coastline of Antarctica provide some of the strongest constraints on the size and shape of the last glacial Antarctic icesheets. The ages of these terraces have could provide powerful information about the timing of retreat of the icesheets. Accurate ¹⁴C dating of the biogenic material on these beaches is, however, complicated by poor knowledge of Southern Ocean reservoir ages during the Holocene. Studies have had to rely on the untested and probably invalid assumption that the best estimate for the pre-bomb reservoir age (~1300 years) represents a constant value throughout the Holocene.

2. Marine sediments in the Southern Ocean provide important information about the paleoceanography of the region but, again, chronology is difficult due to scarcity of information about past reservoir ages. This prevents ready comparison of the timing of events in the Southern Ocean with those elsewhere, so that leads and lags in the climate system cannot be identified.

3. The reservoir age of surface waters is itself a powerful tool with which to assess past ocean circulation. In the same way that the modern distribution of ¹⁴C can be used to assess present ocean circulation (e.g. WOCE), knowledge of past ¹⁴C distributions provide information about past circulation. In the Southern Ocean, variability in ¹⁴C reservoir ages will constrain changes in the rates of upwelling and deep-water formation. Such changes have been hypothesised (Broecker and Sutherland 2000) and, if true, would have significant implications for the mechanisms forcing Holocene climate change.

Estimating reservoir ages relies on finding a marine substrate that can be dated reliably by both ¹⁴C and an alternative chronometer with a precision of ≈100 years or better. In the Southern Ocean, finding such a substrate has proved difficult and existing estimates of the pre-bomb reservoir age are largely based on ¹⁴C dating of museum specimens collected prior to weapons testing in the 1950s. This limited data indicate that, on average, the pre-bomb reservoir effect in the 20th century was ~1300 years (Berkman and Forman 1996). Knowledge of earlier reservoir ages is entirely absent.

In this study, we are using paired radiocarbon and U/Th dates of solitary corals to establish a Holocene history of reservoir ages at two localities within the Ross Sea region. These data will afford us the opportunity to reconstruct Holocene changes in the Southern Ocean radiocarbon reservoir.