Daniel P. Schrag

Toward a Neoproterozoic composite carbon-isotope record

Glacial deposits of Sturtian and Marinoan age occur in the well-studied Neoproterozoic successions of northern Namibia, South Australia, and northwestern Canada. In all three regions, the Marinoan glaciation is presaged by a large negative δ 13 C anomaly, and the cap carbonates to both glacial units share a suite of unique sedimentological, stratigraphic, and geochemical features. These global chronostratigraphic markers are the bases of a new correlation scheme for the Neoproterozoic that corroborates radiometric data that indicate that there were three glacial epochs between ca. 750 and 580 Ma. Intraregional correlation of Neoproterozoic successions in the present-day North Atlantic region suggests that glacial diamictite pairs in the Polarisbreen Group in northeastern Svalbard and the Tillite Group in eastern Greenland were deposited during the Marinoan glaciation, whereas the younger of a pair of glacials (Mortensnes Formation) in the Vestertana Group of northern Norway was deposited during the third (Gaskiers) Neoproterozoic glaciation. Gaskiers-aged glacial deposits are neither globally distributed nor overlain by a widespread cap carbonate but are associated with an extremely negative δ 13 C anomaly. framework for a new, high-resolution model carbon-isotope record for the Neoproterozoic comprising new δ 13 C (carbonate) data from Svalbard (Akademikerbreen Group) and Namibia (Otavi Group) and data in the literature from Svalbard, Namibia, and Oman. A new U-Pb zircon age of 760 ± 1 Ma from an ash bed in the Ombombo Subgroup in Namibia provides the oldest direct time-calibration point in the compilation, but the time scale of this preliminary δ 13 C record remains

Pore Fluid Constraints on the Temperature and Oxygen Isotopic Composition of the Glacial Ocean

Pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical Atlantic indicate that the oxygen isotopic composition (δ18O) of seawater at this site during the last glacial maximum was 0.8 ± 0.1 per mil higher than it is today. Combined with the δ18O change in benthic foraminifera from this region, the elevated ratio indicates that the temperature of deep water in the tropical Atlantic Ocean was 4°C colder during the last glacial maximum. Extrapolation from this site to a global average suggests that the ice volume contribution to the change in δ18O of foraminifera is 1.0 per mil, which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from Barbados corals and terrestrial climate proxies.

Calibrating the Cryogenian

Aging Snowball Earth Earths glacial cycles have varied dramatically over time; at one point glaciers may have covered nearly the entire planet. Correlating various paleoclimate proxies such as fossil and isotope records from that time hinges on the ability to acquire precise age estimates of rocks deposited around the time of this so-called “Snowball Earth.” Macdonald et al. (p. 1241) report new high-precision U-Pb dates of Neoproterozoic strata in the Yukon and Northwest Territories, Canada, to calibrate the timing of carbon isotope variation in rocks from other locations around the globe. Based on the estimated past positions of where these rocks were deposited, glaciers probably extended to equatorial latitudes. The overlap with the survival and, indeed, diversification of some eukaryotes in the fossil record suggests that life survived in localized ecological niches during this global glaciation. A volcanic tuff dated to 716.5 million years ago calibrates the timing of a global glaciation event and eukaryotic survival. The Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.

The oxygen isotopic composition of seawater during the Last Glacial Maximum

High-resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep-sea sediments from sites in the North and South Atlantic. The data provide direct measurements of changes in the isotopic composition of bottom waters during the Last Glacial Maximum (LGM). Results from Ocean Drilling Program (ODP) Site 981 in the North Atlantic, currently bathed in North Atlantic Deep Water (NADW) reproduces previous results from the Ceara and Bermuda Rises, constraining the glacial–interglacial change in δ^(18)O of the deep Atlantic to be 0.7–0.8‰. Results from Site 984, which is located north of Site 981 and at a shallower water depth, yield a similar value (0.8‰), providing insight into the properties of Glacial North Atlantic Intermediate Water (GNAIW). Sites from ODP Leg 177 in the South Atlantic span the modern boundary between northern and southern sources of deep water. Data from the northern site (1088) yield a similar result to sites in the tropical and North Atlantic (0.7‰). At the southern site (1093), located south of the polar front, the change is substantially larger (1.1‰), representing the change in δ^(18)O of southern source waters since the LGM. These results confirm previous estimates that the global average change in δ^(18)O of seawater is 1.0±0.1‰. Hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations, giving further support to these results. At all sites studied, the temperature of the deep ocean during the LGM, calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera, was within 1°C of the freezing point of seawater.

Rapid analysis of high‐precision Sr/Ca ratios in corals and other marine carbonates

A method for rapid determination of high-precision Sr/Ca ratios in scleractinian corals is presented. Using an inductively coupled plasma atomic emission spectrophotometer, samples are corrected for instrument drift using a reference solution, similar to the approach used for analysis of stable isotopes using gas-source mass spectrometry. Further correction for variation of the Sr/Ca ratio with Ca concentration is accomplished using internal standards. The precision, once all corrections have been made, is better than 0.1% (relative standard deviation, 1σ) for samples of similar Ca concentration and better than 0.2% for samples with variable Ca concentrations. This method increases the sample throughput by approximately a factor of 20 relative to thermal ionization mass spectrometry and significantly reduces instrument and per sample costs. Comparison of Sr/Ca data for a coral from the Galapagos Islands with an instrumental temperature record shows excellent agreement and demonstrates the potential for application of this technique to samples of modern and fossil scleractinian corals and other marine carbonates, including foraminifera.

Reconstructing past sea surface temperatures: Correcting for diagenesis of bulk marine carbonate

A numerical model which describes oxygen isotope exchange during burial and recrystallization of deep-sea carbonate is used to obtain information on how sea surface temperatures have varied in the past by correcting measured δ18O values of bulk carbonate for diagenetic overprinting. Comparison of bulk carbonate and planktonic foraminiferal δ18O records from ODP site 677A indicates that the oxygen isotopic composition of bulk carbonate does reflect changes in sea surface temperature and δ18O. At ODP Site 690, we calculate that diagenetic effects are small, and that both bulk carbonate and planktonic foraminiferal δ18O records accurately reflect Paleogene warming of high latitude surface oceans, biased from diagenesis by no more than 1°C. The same is likely to be true for other high latitude sites where sedimentation rates are low. At DSDP sites 516 and 525, the effects of diagenesis are more significant. Measured δ18O values of Eocene bulk carbonates are more than 2% lower at deeply buried site 516 than at site 525, consistent with the model prediction that the effects of diagenesis should be proportional to sedimentation rate. Model-corrections reconcile the differences in the data between the two sites; the resulting paleotemperature reconstruction indicates a 4°C cooling of mid-latitude surface oceans since the Eocene. At low latitudes, the contrast in temperature between the ocean surface and bottom makes the carbonate δ180 values particularly sensitive to diagenetic effects; most of the observed variations in measured δ18O values are accounted for by diagenetic effects rather than by sea surface temperature variations. We show that the data are consistent with constant equatorial sea surface temperatures through most of the Cenozoic, with the possible exception of the early Eocene, when slightly higher temperatures are indicated. We suggest that the lower equatorial sea surface temperatures for the Eocene and Oligocene reported in other oxygen isotope studies are artifacts of diagenetic recrystallization, and that it is impossible to reconstruct accurately equatorial sea surface temperatures without explicitly accounting for diagenetic overprinting.

High-resolution stable isotope records from the Upper Cretaceous rocks of Italy and Spain: Glacial episodes in a greenhouse planet?

High-resolution δ13C and δ18O records from upper Albian to lower Santonian pelagic carbonates of the Contessa Quarry section in Italy exhibit large positive oxygen isotopic excursions of ∼1‰ in the lower Cenomanian and upper Turonian–Coniacian strata. Within the uncertainties of biostratigraphic correlation, these positive excursions appear to correspond to times of large sea-level regressions in global sequence stratigraphic sea-level curves. Several lines of evidence suggest that the major δ18O excursions in Contessa reflect episodes of global cooling and not differential diagenesis. Numerical models of oxygen isotope exchange during diagenesis show that a high contrast in the degree of alteration would be required to produce these signals as artifacts of diagenesis, and lithological data provide no evidence for such large contrasts in the degree of alteration. Furthermore, although precise correlation with a section in the south of Spain is hampered by stratigraphic complexities, the general sequence of major positive δ18O excursions is reproduced. It is unlikely that differential diagenesis would produce similar artifacts in multiple sites. One explanation for the link between episodes of global cooling and sea-level falls is that global cooling events led to polar ice-sheet accumulation, lowering sea level. Although ice-free conditions have been inferred from evidence for a much warmer climate in Late Cretaceous time, our results suggest that the relationship between continental high-latitude ice sheets and overall climatic warmth warrants further examination.

Permanent carbon dioxide storage in deep-sea sediments

Stabilizing the concentration of atmospheric CO2 may require storing enormous quantities of captured anthropogenic CO2 in near-permanent geologic reservoirs. Because of the subsurface temperature profile of terrestrial storage sites, CO2 stored in these reservoirs is buoyant. As a result, a portion of the injected CO2 can escape if the reservoir is not appropriately sealed. We show that injecting CO2 into deep-sea sediments <3,000-m water depth and a few hundred meters of sediment provides permanent geologic storage even with large geomechanical perturbations. At the high pressures and low temperatures common in deep-sea sediments, CO2 resides in its liquid phase and can be denser than the overlying pore fluid, causing the injected CO2 to be gravitationally stable. Additionally, CO2 hydrate formation will impede the flow of CO2(l) and serve as a second cap on the system. The evolution of the CO2 plume is described qualitatively from the injection to the formation of CO2 hydrates and finally to the dilution of the CO2(aq) solution by diffusion. If calcareous sediments are chosen, then the dissolution of carbonate host rock by the CO2(aq) solution will slightly increase porosity, which may cause large increases in permeability. Karst formation, however, is unlikely because total dissolution is limited to only a few percent of the rock volume. The total CO2 storage capacity within the 200-mile economic zone of the U.S. coastline is enormous, capable of storing thousands of years of current U.S. CO2 emissions.

Application of benthic foraminiferal Mg/Ca ratios to questions of Cenozoic climate change

Abstract We investigate the evolution of Cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater (δ18Osw) derived from benthic foraminiferal Mg/Ca ratios to constrain the temperature effect contained in foraminiferal δ18O values. We have constructed two benthic foraminiferal Mg/Ca records from intermediate water depth sites (Ocean Drilling Program sites 757 and 689 from the subtropical Indian Ocean and the Weddell Sea, respectively). Together with the previously published composite record of Lear et al. [Science 287 (2002) 269–272] and the Neogene record from the Southern Ocean of Billups and Schrag [Paleoceanography 17 (2002) 10.1029/2000PA000567], we obtain three, almost complete representations of the δ18Osw for the past 52 Myr. We discuss the sensitivity of early Cenozoic Mg/Ca-derived paleotemperatures (and hence the δ18Osw) to assumptions about seawater Mg/Ca ratios. We find that during the middle Eocene (∼49–40 Ma), modern seawater ratios yield Mg/Ca-derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice-free conditions. Intermediate waters cooled during the middle Eocene reaching minimum temperatures by 40 Ma. The corresponding δ18Osw reconstructions support ice growth on Antarctica beginning by at least 40 Ma. At the Eocene/Oligocene boundary, Mg/Ca ratios (and hence temperatures) from Weddell Sea site 689 display a well-defined maximum. We caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of Mg in benthic foraminifera may be sensitive to factors other than temperature. Throughout the remainder of the Cenozoic, the temporal variability among δ18Osw records is similar and similar to longer-term trends in the benthic foraminiferal δ18O record. An exception occurs during the Pliocene when δ18Osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal δ18O records, which provides a new perspective to the ongoing debate about the stability of the Antarctic ice sheet. Maximum δ18Osw values recorded during the Pleistocene at Southern Ocean site 747 agree well with values derived from the geochemistry of pore waters [Schrag et al., Science 272 (1996) 1930–1932] further highlighting the value of the new Mg/Ca calibrations of Martin et al. [Earth Planet. Sci. Lett. 198 (2002) 193–209] and Lear et al. [Geochim. Cosmochim. Acta 66 (2002) 3375–3387] applied in this study. We conclude that the application of foraminiferal Mg/Ca ratios allows a refined view of Cenozoic ice volume history despite uncertainties related to the geochemical cycling of Mg and Ca on long time scales.

Authigenic Carbonate and the History of the Global Carbon Cycle

The Third Way Because organic carbon contains a larger fraction of the light isotope 12C than inorganic carbonate, variations in the carbon isotopic record of sedimentary rocks are thought to represent changes in the amount of organic carbon buried as sediments (and thus removed from the rest of the carbon cycle). Schrag et al. (p. 540; see the Perspective by Canfield and Kump) suggest that historically a third component was important: authigenic carbonate. Authigenic carbonate is not produced in any appreciable quantity today, but was much more abundant when the level of atmospheric oxygen was low. Carbonate produced in sediment pore fluids played a major role in the carbon cycle in the geological past. [Also see Perspective by Canfield and Kump] We present a framework for interpreting the carbon isotopic composition of sedimentary rocks, which in turn requires a fundamental reinterpretation of the carbon cycle and redox budgets over Earths history. We propose that authigenic carbonate, produced in sediment pore fluids during early diagenesis, has played a major role in the carbon cycle in the past. This sink constitutes a minor component of the carbon isotope mass balance under the modern, high levels of atmospheric oxygen but was much larger in times of low atmospheric O2 or widespread marine anoxia. Waxing and waning of a global authigenic carbonate sink helps to explain extreme carbon isotope variations in the Proterozoic, Paleozoic, and Triassic.