Hagit P. Affek

Pronounced zonal heterogeneity in Eocene southern high-latitude sea surface temperatures

Significance Reconstructions of ancient high-latitude climates can help to constrain the amplification of global warming in polar environments. Climate models cannot reproduce the elevated high-latitude temperature estimates in the Eocene epoch, possibly indicating problems in simulating polar climate change. Widely divergent near-Antarctic Eocene sea surface temperature (SST) estimates, however, question the evidence for extreme warmth. Our analysis of multiple temperature proxies near the Antarctic Peninsula improves intersite comparisons and indicates a substantial zonal SST gradient between the southwest Pacific and South Atlantic. Simulations of Eocene ocean temperatures imply that the formation of deep water in the southwest Pacific partly accounts for this SST gradient, suggesting that climate models underestimate Eocene SSTs in regions where the thermohaline circulation leads to relatively high temperatures. 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.

Warm, not super-hot, temperatures in the early Eocene subtropics

The early Eocene (ca. 55–48 Ma) encompasses one of the warmest intervals of the past 65 m.y. and is characterized by an unusually low equator-to-pole thermal gradient. Recent proxy studies suggest temperatures well in excess of 30 °C even at high latitudes, but confl icting interpretations derived from different types of data leave considerable uncertainty about actual early Eocene temperatures. A robust comparison among new paleotemperature proxies may provide insight into possible biases in their temperature estimates, and additional detail on the spatial distribution of temperatures will further resolve the early Eocene meridional temperature gradient. We use a suite of paleotemperature proxies based on the chemistry of bivalve shell carbonate and associated sedimentary organic matter from the United States Gulf Coastal Plain to constrain climate at a subtropical site during this key interval of Earth history. Oxygen isotope and clumped isotope analyses of shell carbonate and two tetraether lipid analyses of sedimentary organic carbon all yield temperatures of ~27 °C. High-resolution, intraannual oxygen isotope data reveal a consistent, large range of seasonal variation, but clumped isotope data suggest that seasonality is due primarily to precipitation, not to temperature. These paleotemperature estimates are 2–3 °C warmer than the northern Gulf of Mexico today, and generally consistent with early Eocene temperature estimates from other low and mid-latitude locations, but are signifi cantly cooler than contemporaneous estimates from high southern latitudes.

Changes in mixing ratio and isotopic composition of CO2 in urban air from the Los Angeles basin, California, between 1972 and 2003

Atmospheric CO2 mixing ratios and C and O isotopic compositions are reported for the Los Angeles basin in southern California, a region renowned for its air pollution. Air samples collected midday on the Caltech campus in Pasadena, California, contained ∼30 ppm more CO2 in 1998–2003 than in 1972–1973 (averaging 397 ppm in 1998–2003 and 366 ppm in 1972–1973) compared to a 47 ppm change in background air CO2, yet the ranges of the carbon and oxygen isotopic compositions remained essentially constant. Because the 1998–2003 data show a significant progression through time, analysis was done on data from 2002 to 2003 complete calendar years (CO2 mixing ratios increased 41 ppm between 1972 and 1973 and 2002–2003). Both 1972–1973 and 2002–2003 data sets display significant correlation between δ 13C and 1/[CO2] with local CO2 source end-member δ 13C values of −30.9 ± 0.5‰ for 1972–1973 and −29.9 ± 0.2‰ for 2002–2003 (1σ errors). Mass balance calculations explain that this apparently coincidental similarity reflects a change in the relative proportion of natural gas and petroleum products burned in the region combined with a change in the origin, and thus isotopic composition, of the petroleum burned. The δ 13C of the average CO2 inventory in Pasadena can be explained by local addition to background air of 38 ± 4 ppm CO2 in 1972–1973 and 29 ± 3 ppm in 2002–2003 from anthropogenic sources, in seeming contradiction to the known increase in CO2 emissions between these two time periods.

Clumped isotopic equilibrium and the rate of isotope exchange between CO2 and water

The reaction of CO2 hydration/dehydration controls the oxygen isotopic composition in both carbonate minerals and atmospheric CO2 through the exchange of oxygen isotopes with water. The use of δ18O as an environmental indicator typically assumes isotopic equilibrium, namely full oxygen isotope exchange between CO2 and water. Clumped isotopes is a new isotopic tracer that is used in both CaCO3 and atmospheric CO2 and reflects the thermodynamic preference of two heavy isotopes, 13C and 18O in this case (given as Δ47), to “clump” together into one chemical bond at low temperatures. As such, the use of Δ47 as an indicator for temperature relies on the assumption of isotopic equilibrium. The experiments presented here examine the rate in which Δ47 of CO2 that interact with water approaches the equilibrium values. This rate is indistinguishable between Δ47 and δ18O, suggesting that the isotope exchange with water also leads to reorganization of the isotopes among CO2 isotopologues thus controlling the Δ47 values. The direct implication of the temporal link between Δ47 and δ18O is that when one isotopic system shows disequilibrium, either in DIC or in gas phase CO2, so will the other. As CO2 clumped isotope values are independent of the oxygen isotopic composition of the water participating in the reaction, disequilibrium in Δ47 is often identified more readily than in δ18O. The combination of clumped isotopes and oxygen isotopes is therefore likely to elucidate cases of suspected disequilibrium also in δ18O (and vice versa).

Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation.

The stratospheric CO2 oxygen isotope budget is thought to be governed primarily by the O(1D)+CO2 isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO2 isotopologue 16O13C18O, in concert with 18O and 17O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric 16O13C18O, observed as proportions in the polar vortex that are higher than in any naturally derived CO2 sample to date. We show, through photochemical experiments, that lower 16O13C18O proportions observed in the midlatitudes are determined primarily by the O(1D)+CO2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher 16O13C18O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O(1D)+CO2. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO2 or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric 16O13C18O enrichments may impose additional isotopic constraints on biosphere–atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.

Eocene greenhouse climate revealed by coupled clumped isotope - Mg/Ca thermometry

Significance Reconstructing the degree of warming during geological periods of elevated CO2 provides a way of testing our understanding of the Earth system and the accuracy of climate models. We present accurate estimates of tropical sea-surface temperatures (SST) and seawater chemistry during the Eocene (56–34 Ma before present, CO2 >560 ppm). This latter dataset enables us to reinterpret a large amount of existing proxy data. We find that tropical SST are characterized by a modest warming in response to CO2. Coupling these data to a conservative estimate of high-latitude warming demonstrates that most climate simulations do not capture the degree of Eocene polar amplification. Past greenhouse periods with elevated atmospheric CO2 were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO2, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30–36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.