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. 2014 May 6;111(18):6582-7.
doi: 10.1073/pnas.1321441111. Epub 2014 Apr 21.

Pronounced Zonal Heterogeneity in Eocene Southern High-Latitude Sea Surface Temperatures

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Pronounced Zonal Heterogeneity in Eocene Southern High-Latitude Sea Surface Temperatures

Peter M J Douglas et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Paleoclimate studies suggest that increased global warmth during the Eocene epoch was greatly amplified at high latitudes, a state that climate models cannot fully reproduce. However, proxy estimates of Eocene near-Antarctic sea surface temperatures (SSTs) have produced widely divergent results at similar latitudes, with SSTs above 20 °C in the southwest Pacific contrasting with SSTs between 5 and 15 °C in the South Atlantic. Validation of this zonal temperature difference has been impeded by uncertainties inherent to the individual paleotemperature proxies applied at these sites. Here, we present multiproxy data from Seymour Island, near the Antarctic Peninsula, that provides well-constrained evidence for annual SSTs of 10-17 °C (1σ SD) during the middle and late Eocene. Comparison of the same paleotemperature proxy at Seymour Island and at the East Tasman Plateau indicate the presence of a large and consistent middle-to-late Eocene SST gradient of ∼7 °C between these two sites located at similar paleolatitudes. Intermediate-complexity climate model simulations suggest that enhanced oceanic heat transport in the South Pacific, driven by deep-water formation in the Ross Sea, was largely responsible for the observed SST gradient. These results indicate that very warm SSTs, in excess of 18 °C, did not extend uniformly across the Eocene southern high latitudes, and suggest that thermohaline circulation may partially control the distribution of high-latitude ocean temperatures in greenhouse climates. The pronounced zonal SST heterogeneity evident in the Eocene cautions against inferring past meridional temperature gradients using spatially limited data within given latitudinal bands.

Keywords: climate modeling; clumped isotopes; high-latitude climate; organic geochemistry; paleooceanography.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
The Eocene locations of SST proxy sites discussed in the text, shown with the UVic model Eocene paleogeography and sea surface temperatures from a model simulation with 1,600 ppm pCO2. The modeled region of deep-water formation in the Ross Sea is outlined in red. The modeled depth-integrated stream function is also shown; solid and dashed lines indicate clockwise and counter clockwise flow paths, respectively. The proto-Ross and proto-Weddell gyres are labeled with an R and a W, respectively.
Fig. 2.
Fig. 2.
Δ47 vs. estimated growth temperature in modern bivalve samples. Error bars indicate the analytical standard error (1σ SEM) of replicate measurements. With one exception the modern samples agree well with the Δ47–T calibration based on measurements of synthetic calcite (23). The Δ47 value of Laternula elliptica (unfilled red circle) do not agree with the synthetic calcite calibration. This shell is trimineralic and includes vaterite (29), which may account for its divergent Δ47 value. Similar anomalously low Δ47 values have been observed in other cold-water taxa (28, 30).
Fig. 3.
Fig. 3.
(A) Photograph of cross-section of Cucullaea shell 0170C1. Calcite cement is located in the upper right interior of the shell; the darker material filling the rest of the shell is lithified sediment. (B) Comparison of Δ47-derived temperatures for bivalve shells and co-occurring void-filling cement. Cements clearly record a higher temperature than shell carbonate, reflecting the higher temperatures during burial and indicating that temperatures recorded by bivalve shells were not substantially altered during diagenesis.
Fig. 4.
Fig. 4.
(A) SST estimates from the Seymour Island La Meseta Fm. and the East Tasman Plateau (ODP site 1172). Error bars for Δ47 measurements depict analytical uncertainty (typically ±2–4 °C SEM). Calibration error for TEX86H and TEX86L (9) are indicated in the legend. δ18O-derived temperature estimates are calculated using δ18Ow values calculated from clumped isotope derived temperatures (Fig. S2 and SI Text); the black line depicts the mean δ18O-derived temperature and the gray band indicates the range of all values. (B) Temperature differences between Seymour Island and the East Tasman Plateau, as derived from either proxy data or model output. Data points depict differences between East Tasman Plateau temperatures (TEX86L, green; TEX86H, orange) (7) and Seymour Island multiproxy (Δ47 and TEX86L) temperatures. The plotted error bars indicate the combination of the SEM of binned paleotemperatures and the calibration error for TEX86H and TEX86L. The red and blue bands indicate the range of SST differences in the UVic climate model simulations with and without deep-water formation (DWF) in the Ross Sea, respectively. See text for details.
Fig. 5.
Fig. 5.
Comparison of TEX86L and model SSTs for the middle and late Eocene southwest Pacific and South Atlantic. Boxplots indicate the distribution of TEX86L SST estimates from stratigraphic horizons dated to 45 Ma or younger (Fig. S5). The top, center, and bottom of the boxes indicate the upper quartile, median, and lower quartile values, respectively; the outer bars indicate maximum and minimum values; + and * indicate that only middle or late Eocene data are available for the indicated sites, respectively. Red and blue squares indicate UVic model site-specific annual SSTs under different DWF and pCO2 scenarios.

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