OCEAN CIRCULATION DURING THE
LAST GLACIAL MAXIMUM
The ocean’s role in regulating atmospheric carbon dioxide on glacial-interglacial timescales remains a primary unresolved issue in paleoclimatology. Reduced mixing between deep waters may have aided storage of atmospheric CO2 at the Last Glacial Maximum (LGM; 20,000 years ago), but data supporting this idea have remained elusive. Here we use published oxygen and carbon stable isotopic data in a new tracer balance to constrain the circulation and mixing in the deep ocean. We define the use of foraminiferal oxygen isotopes as a conservative tracer and infer the ratio of mass transport to vertical mixing in the abyssal Atlantic. By combining oxygen and carbon stable isotopic data, we also estimate the ratio of biological remineralization to vertical mixing. Our calculations suggest that both ratios were larger at the LGM. The simplest explanation is a reduction in vertical mixing between northern and southern component waters, driven by movement of this watermass boundary away from intense mixing near the seafloor. Our results support the hypothesis that changes in mixing between deep water masses played a role in sequestration of atmospheric carbon dioxide during glacial times.
Curry and Oppo in a seminal paper appeared in Paleocenography in 2005 showed that the vertical gradient in Atlantic Ocean benthic foraminiferal δ13C were stronger during the Last Glacial Maximum than today. We show that the Last Glacial Maximum benthic foraminiferal δ18O profiles also have a larger gradient with water depth than today (Figure 1). At the Brazil Margin (30ºS), the change in Cibicidoides δ18O between 1 and 3 km was approximately 0.2 ‰ larger during the Last Glacial Maximum (Figure 1a). The δ18O gradient is most easily observed in the difference between Last Glacial Maximum and Holocene profiles (Figure 1b). Last Glacial Maximum-Holocene δ18O values above 2 km water depth are generally less than 1.9 ‰, while those below 2 km generally exceed 1.9 ‰. The ‘step’ in the data is more pronounced at the Blake Ridge (30ºN), where it is observed in two genera of benthic foraminifera, Cibicidoides and Uvigerina (Figure 1d). At each location, the Last Glacial Maximum-Holocene δ18O pattern is driven by greater Last Glacial Maximum δ18O below ~1800 m. Last Glacial Maximum δ18O data from the Indian Ocean also show a clear watermass boundary near 1800 m water depth (Kallel et al., 1988) (SOM). The similar depth of this transition at the Brazil Margin and Blake Ridge, and the comparable absolute δ18O value at which it occurs (~ 4.3 ‰) strongly suggests there was a watermass boundary near 2 km in the LGM Atlantic, consistent with inferences from benthic foraminiferal δ13C (Oppo and Lehman, 1993).
Figure 1. A) Brazil Margin Cibicidoides spp. δ18O (solid circles) (Curry and Oppo, 2005), outliers (open circles) and interpolated data points(+). Outliers are defined as those values which exceed ±0.3‰ of the mean defined by adjacent values in water depth. Interpolated values are based on linear interpolation between data points at adjacent water depths. B) Brazil Margin LGM-Holocene δ18O. C) Blake Ridge Cibicidoides spp. δ18O (solid circles) (Keigwin, 2004), outliers (open circles), and interpolated data points (+). Also included are Uvigerina spp. δ18O (grey triangles) (Keigwin, 2004). D) Blake Ridge LGM-Holocene δ18O.
References
Lund, D., J. Adkins and R. Ferrari: Abyssal Atlantic Circulation during the Last Glacial Maximum, Paloeoceanography, in press.
