Karen L. Bice

A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO2 concentrations

foraminiferal d 18 O and Mg/Ca suggests that the ratio of magnesium to calcium in the Turonian-Coniacian ocean may have been lower than in the Albian-Cenomanian ocean, perhaps coincident with an ocean 87 Sr/ 86 Sr minimum. The carbon isotopic compositions of distinct marine algal biomarkers were measured in the same sediment samples. The d 13 C values of phytane, combined with foraminiferal d 13 C and inferred temperatures, were used to estimate atmospheric carbon dioxide concentrations through this interval. Estimates of atmospheric CO2 concentrations range between 600 and 2400 ppmv. Within the uncertainty in the various proxies, there is only a weak overall correspondence between higher (lower) tropical temperatures and more (less) atmospheric CO2. The GENESIS climate model underpredicts tropical Atlantic temperatures inferred from ODP Leg 207 foraminiferal d 18 O and Mg/Ca when we specify approximate CO2 concentrations estimated from the biomarker isotopes in the same samples. Possible errors in the temperature and CO2 estimates and possible deficiencies in the model are discussed. The potential for and effects of substantially higher atmospheric methane during Cretaceous anoxic events, perhaps derived from high fluxes from the oxygen minimum zone, are considered in light of recent work that shows a quadratic relation between increased methane flux and atmospheric CH4 concentrations. With 50 ppm CH4, GENESIS sea surface temperatures approximate the minimum upper ocean temperatures inferred from proxy data when CO2 concentrations specified to the model are near those inferred using the phytane d 13 C proxy. However, atmospheric CO2 concentrations of 3500 ppm or more are still required in the model in order to reproduce inferred maximum temperatures.

Jiggling the tropical thermostat in the Cretaceous hothouse

Modern open-ocean sea-surface temperatures rarely exceed ;28‐29 8C, and the same has been thought to represent a rough maximum for past tropical climates. However, new isotopic estimates from the uppermost Cenomanian in the tropical western North Atlantic suggest that mixed-layer temperatures reached ;33‐34 8 C( 62 8C) during the middle Cretaceous hothouse. Uppermost Cenomanian tropical sea-surface temperatures may have been as much as 4‐7 8C warmer than the highest modern mean annual temperatures. Such extreme conditions suggest that warm tropical oceans could have driven substantially intensified atmospheric heat transport near the Cenomanian-Turonian boundary. The tropical ‘‘thermostat’’ was set higher than today, challenging the hypothesis of tropical climate stability.

Ocean stagnation and end-Permian anoxia

Ocean stagnation has been invoked to explain the widespread occurrence of organic-carbon–rich, laminated sediments interpreted to have been deposited under anoxic bottom waters at the time of the end-Permian mass extinction. However, to a first approximation, stagnation would severely reduce the upwelling supply of nutrients to the photic zone, reducing productivity. Moreover, it is not obvious that ocean stagnation can be achieved. Numerical experiments performed with a three-dimensional global ocean model linked to a biogeochemical model of phosphate and oxygen cycling indicate that a low equator to pole temperature gradient could have produced weak oceanic circulation and widespread anoxia in the Late Permian ocean. We find that polar warming and tropical cooling of sea-surface temperatures cause anoxia throughout the deep ocean as a result of both lower dissolved oxygen in bottom source waters and increased nutrient utilization. Buildup of quantities of H2S and CO2 in the Late Permian ocean sufficient to directly cause a mass extinction, however, would have required large increases in the oceanic nutrient inventory.

Extreme polar warmth during the Cretaceous greenhouse? Paradox of the late Turonian δ18O record at Deep Sea Drilling Project Site 511

[1] Oxygen isotope data for upper Turonian planktonic foraminifera at Deep Sea Drilling Project Site 511 (Falkland Plateau, 60°S paleolatitude) exhibit an ∼2‰ excursion to values as low as -4.66‰ (Vienna Peedee belemnite standard; PDB) coincident with the warmest tropical temperature estimates yet obtained for the open ocean. The lowest planktonic foraminifer δ 1 8 O values suggest that the upper ocean was as warm as 30-32°C. This is an extraordinary temperature for 60°S latitude but is consistent with temperatures estimated from apparently coeval mollusc δ 1 8 O from nearby James Ross Island (65°S paleolatitude). Glassy textural preservation, a well-defined depth distribution in Site 511 planktonics, low sediment burial temperature (∼32°C), and lack of evidence of highly depleted pore waters argue against diagenesis (even solid state diffusion) as the cause of the very depleted planktonic values. The lack of change in benthic foraminifer δ 1 8 O suggests brackish water capping as the mechanism for the low planktonic δ 1 8 O values. However, mixing ratio calculations show that the amount of freshwater required to produce a 2‰ shift in ambient water would drive a 7 psu decrease in salinity. The abundance and diversity of planktonic foraminifera and nannofossils, high planktonic:benthic ratios, and the appearance of keeled foraminifera argue against lower-than-normal marine salinities. Isotope calculations and climate models indicate that we cannot call upon more depleted freshwater δ 1 8 O to explain this record. Without more late Turonian data, especially from outside the South Atlantic basin, we can currently only speculate on possible causes of this paradoxical record from the core of the Cretaceous greenhouse.

Continental runoff and early Cenozoic bottom-water sources

The dominance of warm saline bottom water during the mid-Cretaceous and the early Cenozoic has been inferred from sea-floor sediment records, an interpretation supported by early ocean general circulation model experiments. Thermohaline circulation depends in part on upper ocean salinities; however, early ocean models neglected continental runoff, a potentially critical factor in the salinity budget of the surface ocean. Our early Eocene ocean model sensitivity tests show that model deep-water sources can be enhanced, diminished, or turned off by varying the treatment of continental runoff in the atmosphere-ocean moisture flux calculation. Failure to treat surface runoff adequately thus has important implications for the simulation of thermohaline flow and formation of warm saline bottom water. Variations in runoff could have led to rapid changes in the relative importance of high-latitude versus subtropical deep water, such as may have occurred during the late Paleocene–early Eocene boundary interval (≈ 53.6–56.2 Ma).

LATE PALEOCENE ARCTIC OCEAN SHALLOW-MARINE TEMPERATURES FROM MOLLUSC STABLE ISOTOPES

Late Paleocene high-latitude (80°N) Arctic Ocean shallow-marine temperatures are estimated from molluscan δ18O time series. Sampling of individual growth increments of two specimens of the bivalve Camptochlamys alaskensis provides a high-resolution record of shell stable isotope composition. The heavy carbon isotopic values of the specimens support a late Paleocene age for the youngest marine beds of the Prince Creek Formation exposed near Ocean Point, Alaska. The oxygen isotopic composition of regional freshwater runoff is estimated from the mean δ18O value of two freshwater bivalves collected from approximately coeval fluviatile beds. Over a 30 – 34‰ range of salinity, values assumed to represent the tolerance of C. alaskensis, the mean annual shallow-marine temperature recorded by these individuals is between 11° and 22°C. These values could represent maximum estimates of the mean annual temperature because of a possible warm-month bias imposed on the average δ18O value by slowing or cessation of growth in winter months. The amplitude of the molluscan δ18O time series probably records most of the seasonality in shallow-marine temperature. The annual temperature range indicated is approximately 6°C, suggesting very moderate high-latitude marine temperature seasonality during the late Paleocene. On the basis of analogy with modern Chlamys species, C. alaskensis probably inhabited water depths of 30–50 m. The seasonal temperature range derived from δ18O is therefore likely to be damped relative to the full range of annual sea surface temperatures. High-resolution sampling of molluscan shell material across inferred growth bands represents an important proxy record of seasonality of marine and freshwater conditions applicable at any latitude. If applied to other regions and time periods, the approach used here would contribute substantially to the paleoclimate record of seasonality.

Warm climate dynamics

Several long-held notions about warm climate dynamics are not supported by quantitative studies. We present an attempt to move toward a better foundation for reconstructions of climate processes and climate change mechanisms during the early Paleogene warm interval. We cite examples from both coupled model experiments of future climate change and general circulation model sensitivity studies that employ boundary conditions appropriate to the late Paleocene–early Eocene earth.

Earth science: Baked Alaska

The warming of the Earths climate more than 50 million years ago is as yet unexplained. Now the finger points to the heating of sediment in the Gulf of Alaska as an important source of the greenhouse gas methane.

“Warm saline bottom water” during the LPTM?

Paleogene records have frequently been interpreted as reflecting changes in the relative importance of high latitude deep water source(s) and an inferred lowto mid-latitude source of warm saline bottom water (WSW), sometimes with abrupt climate change attributed to a sudden “reversal” of the thermohaline circulation. This has occurred despite the fact that the true nature of the vertical structure of any Paleogene ocean is poorly constrained and little direct evidence exists to indicate formation of bottom water in a low latitude region. That the extreme global climatic changes of the late Paleocene thermal maximum (LPTM) may have been caused by initiation or dominance of WSW provides strong motivation for better establishing the vertical watermass structure of the Paleocene/ Eocene (P/E) boundary interval and for ocean modeling designed to establish whether a source of WSW is fundamentally possible and under what conditions one might exist. Potentially, the most reliable indicator of relative bottom water age and proximity to source is the carbon isotopic composition of benthic foraminifera. The interpretation of the best available carbon records indicates that, if some substantial component of bottom waters was formed outside the polar regions during the Paleogene, this is most likely to have occurred at or near the P/E boundary. Based on the comparison of δC of benthic foraminifera at Pacific Ocean Site 577 with North and South Atlantic Ocean sites, young, nutrient-depleted water occurred in the eastern South Atlantic south of Walvis Ridge during the late Paleocene and early Eocene, but this Antarctic source was reduced at the P/E boundary. Benthic foram and ostracod studies on Maud Rise, in the Atlantic sector Southern Ocean, suggest that the region was “close” to a source of deep water from at least the Maastrichtian through the Paleogene, but that this source was greatly diminished or stopped during the boundary interval. Numerous investigators cite evidence consistent with abrupt bottom water warming within the depth range 900–3000 m at the LPTM, usually interpreted as a response to a change in the dominant bottom water source. The rapid global LPTM benthic extinction and the post-extinction biogeography are believed to be consistent with the initiation or enhancement of a strong low latitude bottom water component. Black and gray shales, indicative of organic matter preservation in O2-depleted bottom waters, the nature of the benthic biotic turnover, and the tendency for efficient nutrient utilization during low-latitude convection further support the plausibility of WSW during the boundary interval. Past ocean modeling efforts have not supported the concept of a Tethyan-derived deep water source. However, because of the sensitivity of thermohaline circulation to moisture fluxes and the large degree of uncertainty in this boundary condition for any ancient interval, whether a strong WSW component has existed at all, or has existed as either a steady-state or a transient condition remains unknown. It has therefore become necessary to examine the question of WSW using an intentionally “constraining” approach, in which lowto mid-latitude convection is forced to occur in the