PP34A-01 16:00h
Impact of Atmospheric CO2 and Galactic Cosmic Radiation on Phanerozoic Climate Change and the Marine d18O Record
A new model is developed and applied to simulate the Phanerozoic evolution of seawater composition (dissolved Ca, Sr, dissolved inorganic carbon, alkalinity, pH, d18O), marine carbonates (Sr/Ca, 87Sr/86Sr, d13C, d18O), atmospheric CO2 and surface temperature. The marine carbonate records (Sr/Ca, 87Sr/86Sr, d13C) are used to reconstruct changes in volcanic/tectonic activity and organic carbon burial over the Phanerozoic. Seawater pH is calculated assuming saturation with respect to calcite and considering the changing concentration of dissolved Ca documented by brine inclusion data. Surface temperatures are calculated using the GEOCARB III approach considering also the changing flux of galactic cosmic radiation (GCR). It is assumed that GCR cools the surface of the Earth via enhanced cloud formation at low altitudes. The d18O of marine carbonates is calculated considering the changing isotopic composition of seawater, the prevailing surface temperatures and seawater pH. Repeated model runs showed that the trends observed in the marine d18O record can only be reproduced by the model if GCR is allowed to have a strong effect on surface temperature (Wallmann 2004, G-cubed, 5(1), doi:10.1029/2003GC000683). The climate evolution predicted by the model is consistent with the geological record. Warm periods (Cambrian, Devonian, Triassic, Cretaceous) are characterized by low GCR levels. Cold periods during the late Carboniferous to early Permian and the late Cenozoic are marked by high GCR fluxes and low pCO2 values. The major glaciations occurring during these periods are the result of carbon cycling processes causing a draw-down of atmospheric CO2 and a coevally prevailing dense cloud cover at low-altitudes induced by strong GCR fluxes. The two moderately cool periods during the Ordovician - Silurian and Jurassic - early Cretaceous are characterized by both high pCO2 and GCR levels so that greenhouse warming compensated for the cooling effect of low-altitude clouds. The very high Jurassic d18O values observed in the geological record are caused by low pH values in surface waters rather than cold surface conditions.
PP34A-02 16:15h
Aragonite production in calcite seas: effect of seawater Mg/Ca ratio on the calcification and growth of the calcareous algae {\it Penicillus}, {\it Halimeda} and {\it Udotea}
Stanley and Hardie (1998, 1999) have shown that secular variation in the Mg/Ca ratio of seawater throughout the Phanerozoic would have subjected the aragonite-producing Codiacean algae to three transitions between the so-called calcite (molar Mg/Ca $<$ 2) and aragonite (molar Mg/Ca $>$ 2) seas, since their origin in the Ordovician (Roux, 1991). They assert that major sediment production by Codiacean algae in recent tropical seas is permitted by the molar Mg/Ca ratio of modern seawater ($\sim$5.2) remaining within the range of aragonite seas (molar Mg/Ca $>$ 2). To test this hypothesis, three major sediment producing Codiacean algae, {\it Penicillus capitatus}, {\it Halimeda monile} and {\it Udotea flabellum}, were grown in three artificial ancient seawaters, corresponding to "calcite seas" (molar Mg/Ca = 1.0), "aragonite seas" (molar Mg/Ca = 5.2) and a boundary composition (molar Mg/Ca = 2.5). Significantly, the {\it Penicillus} and {\it Udotea} specimens maintained their aragonitic mineralogy in each of the artificial seawaters, suggesting either that the algae pump cations to create an internal aragonite nucleation field or employ organic templates specifying the nucleation of the aragonite polymorph (Borowitzka 1984). The {\it Halimeda} specimens also produced aragonite in the aragonite and boundary seawaters, but failed to grow at all in the calcite seawater. Linear growth rates, primary productivity and calcification decreased with reductions in ambient Mg/Ca. A stress-strain analysis of the {\it Penicillus} thalli revealed that their stiffnesses also decreased with Mg/Ca. The reduced calcification of the algae grown in the calcite and boundary seawaters is probably due to the kinetic difficulty of precipitating aragonite from seawater which does not favor its nucleation. The decreased rates of linear growth and primary production were probably caused by reductions in CO$_{2}$ available for photosynthesis (CO$_{2}$ + H$_{2}$O = CH$_{2}$O + O$_{2}$) due to the reduction in calcification (2HCO$_{3}$ + Ca = CaCO$_{3}$ + CO$_{2}$ + H$_{2}$O). The decrease in {\it Penicillus} thallus stiffness is probably due to reductions in calcification and primary production. This study suggests that producing aragonite in seawater outside of the aragonite + high-Mg calcite nucleation field would have reduced the competitiveness of these algae, made them more susceptible to predation and decreased their contribution to carbonate sedimentation. These findings support Stanley and Hardie's (1998, 1999) empirical evidence that changing Mg/Ca ratios in the oceans have had a significant impact on the major calcifying reef builders and sediment producers throughout the Phanerozoic.
PP34A-03 16:30h
Phanerozoic Sedimentary C$_{org}$:P Ratios and Paleocean Ventilation
Phosphorus is probably the major limiting nutrient on marine primary productivity at geological timescales, although other elements (e.g., N, Fe) can be transiently biolimiting. It has been inferred that remineralization of organic P and upwelling of nutrient-rich deepwaters have the potential to stimulate primary productivity, and that measurements of sedimentary (C:P)$_{org}$ ratios in ancient sediments can yield insights on nutrient fluxes and productivity levels in paleoceans. However, recent work has shown that (1) sedimentary (C:P)$_{org}$ ratios exceed the Redfield ratio (106:1) in most environments as a result of preferential bacterial destruction of labile P-bearing compounds, and (2) the inorganic P fractions associated with Fe-bound and authigenic phosphate phases in anoxic sediments are largely of organic derivation, representing remineralized P that has been fixed through redox-related processes. These insights suggest that the most useful measure of nutrient regeneration is the nondetrital P fraction of sediments, a variable that is rarely determined but that can be adequately proxied by total P in anoxic facies, in which the detrital P fraction is typically small. In this study, C$_{org}$:P ratios were determined for 60 anoxic facies of Cambrian through Recent age. The Recent facies yield a mean C$_{org}$:P of 65 25, values similar to those for most anoxic facies of Mississippian and younger age. In contrast, many Cambrian-Devonian anoxic facies yield substantially higher C$_{org}$:P ratios, with the highest values (>400) in the Middle-Upper Cambrian and Middle-Upper Devonian. For the Phanerozoic as a whole, the logarithmic mean C$_{org}$:P ratio declines by a factor of four, from ~260:1 in the Cambrian to 65:1 in the Recent, a statistically robust result. Given the importance of redox controls on sedimentary P retention at a local scale, this pattern can be interpreted as a record of benthic redox conditions through time, i.e., paleocean ventilation. Although Recent anoxic facies are oxygen-depleted at present (i.e., an instantaneous condition), their relatively low C$_{org}$:P ratios suggest that, on a time-averaged basis at timescales associated with sedimentary P retention (i.e., 10$^{3}$-10$^{4}$ y), these environments are not nearly as anoxic as their Early-Middle Paleozoic counterparts. Anoxic facies of all ages experience episodic "freshening" events, in which small quantities of oxygen from surface waters are transferred below the chemocline by storm mixing, turbidite flows, or overspill into silled basins. Such freshening events have imparted an "oxic" C$_{org}$:P signature to Recent anoxic facies but not to Cambrian-Devonian anoxic facies. The key difference probably lies in atmospheric O$_{2}$ levels, which may have been sufficiently low during the Cambrian-Devonian that "freshening" events did little to mitigate oxygen-poor conditions in contemporaneous deepwaters. The abrupt mid-Paleozoic decline in sedimentary C$_{org}$:P ratios can be attributed to burial of large quantities of organic matter as marine black shales during the Devonian and as freshwater coals during the Carboniferous, effecting a permanent increase in atmospheric pO$_{2}$ and improved deep ocean ventilation.
PP34A-04 16:45h
Paleoclimate Controls on Pennsylvanian Cyclostratigraphy
Cyclic deposition of Pennsylvanian strata in the United States (US) is generally attributed to eustatic changes in sea level driven by repetitive fluctuations of ice volume in the southern hemisphere of Pangea. Although changes in ice volume may account for eustasy, global paleoclimate cycles appear to best explain temporal changes in the lithostratigraphy of Pennsylvanian cycles in low paleolatitudes of Pangea. Low paleolatitude Pennsylvanian paleoclimate cycles are indicated by the following repetitive stratigraphic successions: 1) continental-scale mineral paleosols and low sediment supply all indicate a relatively humid climate during low stands, 2) histosol formation (coal) in equatorial Pangea (eastern and central US) indicate a wet doldrums belt during low stands, 3) tidal to intertidal deposits overlying paleosols are indicative of the onset of sea level rise, 4) subsequent shallow water black shale deposits in basin centers suggest minimal circulation of epeiric seas and very weak surface winds during the early to mid stages of transgression, 5) increases in siliciclastic sediment supply and the onset of prograding deltas in the east and eolian deposition in the western U. S. as sea level rose suggests climate drying, and increasing wind speeds, and 6) maximum fluvial influx in equatorial regions (eastern US), open marine limestones and evaporites (central and western US) suggest maximum seasonality, maximum dryness, increased wind speeds, and maximum wind-driven circulation in epeiric seas during high stands. The temporal changes within Pennsylvanian depositional cycles in North America appear to be best explained by a global paleoclimate model as follows: 1) the low latitudes of Pangea were wettest during glacial low stands because of a relatively stable low-pressure rainy belt (doldrums) and a stable intertropical convergence zone (ITCZ) in response to high pressure over the ice caps and nearly stationary polar fronts that confined the ITCZ to low latitudes, 2) as the southern hemisphere ice melted and sea level rose, the doldrums low pressure belt gave way to seasonal swings of the ITCZ in response to seasonal heating of air masses over both northern and southern hemisphere land masses, 3) high stands were coeval with maximum climate drying in the low paleolatitudes of Pangea because seasonal landmass heating in both hemispheres intensified north-south swings of the ITCZ during interglacials; rainfall was limited to two relatively weak rainy seasons coincident with the passage of the ITCZ.
PP34A-05 17:00h
Irrepressible El Nino: Perspectives on ENSO and Climate Change From the Deep Past.
The tropical thermocline is shallow enough in today's climate to permit upwelling of cold, subthermocline water on the equator. As a consequence, the tropical oceans can transport a significant amount of heat poleward; furthermore, interaction between upwelling rate, SST and surface wind stress leads to strong interannual fluctuations of equatorial SST, the phenomenon known as ENSO. This is not a necessary state of affairs, however: an alternative regime is conceivable where no upwelling, heat transport or ENSO variability occur. A transition into such a regime would clearly have major consequences for global climate. Can increased radiative forcing drive such a transition? Warm climates of the deep past provide an excellent context in which to explore this question. We present results from a suite of deep-paleoclimate simulations using NCAR's fully-coupled Climate System Model (CSM), which show not only that ENSO survives even in these warm climates, but in fact plays an even bigger global role than at present. These results are supported by comparison with annually-resolved geological proxy data for the Eocene case. Finally, we present a simple model of the equatorial Pacific which facilitates qualitative understanding of the basic physical mechanisms underpinning these results.
PP34A-06 17:15h
Understanding the Causes of Mid-Cretaceous Warming: Implications of Terrestrial Carbonate Isotope Data on Latent Heat Transport and Methane Fluxes During the Albian
The distribution of pedogenic spherulitic siderite (sphaerosiderites) and calcareous paleosols provide evidence of changes in the Albian hydrologic cycle. These pedogenic deposits attest to intensified precipitation in the tropics and mid to high latitudes and enhanced aridity in the dry subtropical belts. The isotopic compositions of the pedogenic minerals provide constraints for an isotope mass balance model of paleolatitudinal changes in the \delta$^{18}$O of Albian paleoprecipitation, and suggest that Albian precipitation rates were 156-220 % greater in the mid-latitudes [2600-3300 mm/yr], and 99 % greater at high latitudes [550 mm/yr]. These higher precipitation rates were sustained by a 76-96 % increase in evaporation fluxes, principally focused in the tropics and subtropics. Comparison of modeled Albian and modern P-E curves suggest amplification of the Albian moisture deficit between 7.5 and $30\deg$N latitude (up to 65% greater), and amplified Albian moisture surplus in the mid to high latitudes (up to 45% greater). The tropical moisture deficit is calculated to represent an average heat loss of approximately 74 W/m$^{2}$ at $10\deg$N paleolatitude (present 16.5 W/m$^{2}$), an average heat gain of approximately 83 W/m$^{2}$ at $45\deg$N (present 23 W/m$^{2}$); and an average heat gain of 19 W/m$^{2}$ at $75\deg$N (present 4 W/m$^{2}$). These quantitative estimates of increased poleward heat transfer by H$_{2}$O vapor during the mid-Cretaceous greenhouse warming help to explain the reduced equator-to-pole temperature gradients. Sphaerosiderites are widely distributed, and often formed under methanogenic conditions. Their \detla$^{13}$C values clearly indicate that methane fluxes from Albian wetland soils of the tropics and the mid-to-high latitudes were significantly higher than present day fluxes. Coupled with episodic clathrate releases, pedogenic methane production may provide an alternative to CO$_{2}$ as the sole driver on warm conditions of the Albian greenhouse world.
PP34A-07 17:30h
The Cenomanian-Turonian Boundary Event: Linkage of High-Resolution Terrestrial and Marine Records of a Major Climate Perturbation During Peak Greenhouse Conditions.
Interdisciplinary studies of paleoclimate provide a critical source of information about the nature and magnitude of changes in the natural climate system, of thresholds and feedbacks in the biogeochemical cycles that modulate climate, and of the biological consequences of extreme climate transitions and states. Unfortunately, many studies of ancient climate are constrained by low temporal resolution, diminished reliability of proxy data, and a lack of information about linkages between different components of the climate system. This talk will summarize recent improvements in the temporal resolution of biogeochemical and paleobiological data across the marine record of the Cenomanian-Turonian Boundary Event (CTBE), and extension of that record into the terrestrial realm of the Western Interior of North America. The CTBE is hypothesized to represent a brief interval of global marine organic carbon burial that occurred during the peak Cretaceous greenhouse. It is thought to have caused an oscillation in pCO$_{2}$ comparable to, or greater in magnitude than glacial-interglacial cycles. This is believed to have caused transient cooling followed by return to maximum Cretaceous warmth with major impacts on the ecology and evolution of terrestrial ecosystems. Efforts to quantify the pCO$_{2}$ effect, to assess available paleoclimate indicators, and to evaluate changes in terrestrial ecosystems across this event have been limited by the constraints mentioned above. However, a number of recent advances are facilitating a new generation of paleoclimatic analysis of the CTBE : 1) development of an orbital time scale for the C-T stratotype in central Colorado with average temporal resolution of 8 kyrs; 2) use of this time scale to calculate burial fluxes for key paleoenvironmental proxies; 3) export of the time scale to a coeval terrestrial section containing fossil whole plant and cuticle material based on high-resolution lithostratigraphic, biostratigraphic and chemostratigraphic correlation; 4) development of new high-resolution d13Corg records for both the marine (average 5 cm resolution; bulk TOC) and terrestrial (average 40 cm resolution; coal, charcoal, and cuticle) sections; and 5) preliminary pCO$_{2}$ estimates based on stomatal index analysis of fossil plant cuticle collected across the CTBE interval. These linked, high-resolution data provide new constraints on the timing and response of terrestrial and marine biogeochemical cycles and ecosystems during the CTBE and thus allow leading mechanistic hypotheses for the event to be re-evaluated.
PP34A-08 17:45h
Linked Paleosol and Model-based Reconstructions of Paleoprecipitation During Episodes of Extreme Global Warmth: Early Eocene and Middle Cretaceous Greenhouse States Compared
The late Paleocene-early Eocene Earth has been characterized as having ice-free poles inhabited by mammals, reptiles and deciduous forests, with globally averaged surface temperatures 2-4°C greater than today. This episode is considered to have been the warmest of the Cenozoic and is perhaps second only to the middle Cretaceous greenhouse as a period of extreme global warmth during the past 100 million years. This study focuses on an oxygen isotopic record of paleoprecipitation derived from early Eocene paleosol siderite spherules obtained along a paleolatitudinal transect from Texas to Alaska, though data obtained from coeval deposits in France, Spain and Australia are considered. The data are used to benchmark a stable isotope tracer version of the GENESIS v. 2 general circulation model and are compared to similar paleosol-model results for the middle Cretaceous Earth. The early Eocene oxygen isotope profile displays a pronounced south to north depletion, an observation that is very similar to a transect of middle Cretaceous paleosols; both profiles are depleted relative to similar modern latitudinal distributions. These results indicate that early Eocene paleotemperatures may have been as warm as those reconstructed for the middle Cretaceous and provide evidence for an amplified atmospheric hydrologic cycle during these episodes of extreme warmth. Our modeling results support these conclusions and indicate that early Eocene and middle Cretaceous Precipitation minus Evaporation (P-E) profiles were substantially altered compared to modern profiles: in tropical and middle latitude regions, P-E values are 2-3X greater than today, whereas in the subtropics, P-E values are up to 3X less than today. Some worst-case climate model scenarios predict future temperatures similar to those hind cast for the middle Cretaceous greenhouse state. However, most researchers conclude that middle Cretaceous climate reconstructions are irrelevant to considerations of near-term climate change due to substantial differences between Cretaceous and modern land-sea distributions. We submit that the overall geographic similarity between the Paleocene-Eocene and modern worlds as well as the similarity between our Cretaceous and Eocene climate reconstructions suggests that the reconstructions may be meaningful as potential worst-case scenarios for Earth's globally-warmed climate future. Our reconstructions further suggest that substantial shifts in the distribution of precipitation can be anticipated under conditions of extreme greenhouse warmth.