PP33C-01 INVITED
Can lipid D/H ratios be a quantitative proxy for aridity?
D/H fractionations within the hydrological cycle respond strongly to climatic variables, including relative humidity and evaporation. We are investigating whether the D/H composition of leaf-wax lipids can provide a quantitative proxy for aridity. At the plant level, the isotopic fractionation between source water and plant lipids records the combined effects of evapotranspiration and biosynthesis. However in arid regions many plants employ a variety of drought avoidance strategies that can substantially influence the isotopic composition of leaf water. Also the seasonality and possible temperature dependence of biosynthetic fractionations in drought-adapted plants has not yet been examined. Strong temporal variations in both precipitation and growth rate add additional complexity. To deconvolve such factors, we are measuring the delta-D of precipitation, stream and groundwater, xylem water, leaf water and leaf wax lipids. Samples are collected from a variety of species, with different drought tolerance strategies, and from a range of climatic regimes. We also monitor the effects of seasonal, spatial and experimental variations in environmental conditions on the D/H ratio of plant leaf wax lipids. Preliminary data from this sampling program will be presented at the meeting.
PP33C-02 INVITED
D/H Ratios in Lipid Biomarkers Record Tropical Pacific Climate Variations Since the Last Glacial Period
Hydrogen isotope ratios in lipids from algae and aquatic plants record the isotopic composition of water with high fidelity. Though a large and variable D/H fractionation during lipid biosynthesis occurs between species and lipid classes, and environmental conditions may influence this fractionation in some instances, robust reconstructions of hydrologic conditions are possible provided that a suitable set of lipids are analyzed. Here we present hydrogen isotopic data from lipid biomarkers in tropical Pacific lake, ocean and hypersaline basin sediments spanning the last 30,000 years that indicate large changes in temperature and rainfall between the LGM and the Holocene, and throughout the entire Holocene period.
PP33C-03
Global warming leads the carbon isotope excursion at the Paleocene-Eocene thermal maximum
The prominent negative carbon isotope excursion (CIE) at the Paleocene-Eocene thermal maximum (55.5 Ma) is generally accepted to reflect a transient, massive input of isotopically light carbon into the ocean-atmosphere system. Many authors have assumed that this carbon led to pronounced global greenhouse warming. Here we show, from an expanded record in New Jersey, that both the onset of the global abundance of the subtropical dinoflagellate Apectodinium and surface-ocean warming as recorded by TEX$_{86}$ preceded the CIE by several thousands of years. The offset between Apectodinium and the CIE was confirmed in other sites from New Jersey, the North Sea and New Zealand. The approximately 3 kyrs time lag between the onset of warming and the CIE is consistent with the expected lag between bottom water warming and submarine methane hydrate dissociation, suggesting that the latter mechanism indeed caused the CIE.
PP33C-04
The Cretaceous Thermal Maximum and Oceanic Anoxic Event 2 in the Tropics: Sea-Surface Temperature and Stable Organic Carbon Isotopic Records from the Equatorial Atlantic
Oceanic Anoxic Event 2 (OAE-2) occurring during the Cenomanian/Turonian transition, is evident from a global positive stable carbon isotopic excursion and presumably represents the most extreme carbon cycle perturbation of the last 100 Myr. However, the impact of this major perturbation on and interaction with global climate remains unclear. OAE-2 occurred in the mid-Cretaceous, a time in Earth history characterized by extreme global warmth culminating in the so-called Cretaceous thermal maximum. Thus, records of paleo-sea surface temperatures (SSTs) from the mid-Cretaceous oceans are particularly important for understanding greenhouse climate conditions. We will present new high-resolution SST-records based on an organic proxy, the TetraEther indeX of 86 carbon atoms (TEX$_{86}$), and $\delta$$^{18}$O of excellently preserved, ''glassy'' planktic foraminifera, combined with stable organic carbon isotopes generated from marine black shales located offshore Suriname/French Guiana (ODP Site 1260) and Senegal (DSDP Site 367). \\At Site 1260 a good match between conservative SST estimates from TEX$_{86}$ and $\delta$$^{18}$O is observed. Late Cenomanian SSTs in the equatorial Atlantic Ocean (~33$\deg$C) were substantially warmer than today (~27-29$\deg$C) and the onset of OAE-2 coincided with a rapid shift to an even warmer (~35-36$\deg$C) regime. Within the early stages of OAE-2 a marked (~4$\deg$C) cooling is observed. However, well before the termination of OAE-2, the warm regime was re-established and persisted into the Turonian. Our findings corroborate the view that the C/T-transition represents the onset of peak Cretaceous warmth, that mid-Cretaceous warmth can be attributed to high levels of atmospheric CO$_{2}$ and that major OAEs were capable of triggering global cooling through the negative feedback effect of organic carbon burial-led CO$_{2}$-sequestration. However, the factors that gave rise to the observed shift to a warmer climate regime at the onset of OAE-2 were sufficiently powerful that they were only briefly counterbalanced by high rates of carbon- burial attained during OAE-2.\\ The latter becomes even more evident when our detailed dual-proxy SST-records from ODP Site 1260 are compared to the long-term evolution of mid-Cretaceous tropical SSTs at Demerara Rise. Here, we monitored the Albian to Santonian SST-history of the western equatorial Atlantic by generating a solely TEX$_{86}$ based combined record that spans the entire Cretaceous black shale sequence at ODP Sites 1258 and 1259 in meter- scale resolution, which was recently completed and will be presented here for the first time. Once established the extreme warm climate regime (characterized by averaged tropical SSTs exceeding 35$\deg$C) lasted well into the Coniacian at Demerara Rise. However, during the Turonian, several pronounced but relatively short-lived cooler intervals punctuate this otherwise remarkably stable interval of extreme tropical warmth. This observation shows that rapid tropical SST-changes occurred also during the Cretaceous thermal maximum, and implies that even the mid-Cretaceous ''super-greenhouse'' climate may have been less stable than previously thought.
PP33C-05 INVITED
Processes Controlling the Dissolved Inorganic Phosphate Concentration in the Ocean Over Geological Timescales
The mean oxygen concentration of the world ocean, and thereby the oceans susceptibility to develop anoxic conditions, is at any point in time dependent on multiple factors. One basic variable considered to be related to oxygen demand in the deep ocean is the nutrient inventory (dissolved inorganic phosphate (DIP) and/or nitrate etc) by its influence on new production of sinking organic matter. Seeking to understand anoxia in the world ocean it is therefore pertinent to ask what processes control the nutrient inventory and how much it could have changed through the Phanerozoic? Recently we have attempted a quantification of the influence that sea level change and shelf area extent exerts on the DIP inventory, marine productivity and burial of organic carbon (Bjerrum et al., 2006). The simple biogeochemical model explicitly considers the seafloor � surface area distribution of Earth as a function of elevation and the burial efficiency now as a function of siliciclastic sedimentation rate, while the input flux of phosphorus is held constant. Based on the model results we find that sea level rise on time scales longer than ~100 kyr results in a significant decreased nutrient inventory of the ocean because of the greater sedimentation rate and burial efficiency in expanded shelf areas. The reduced nutrient inventory results in decreased productivity which eventually causes an oxygenation of the global ocean. Additional processes can result in changes in the nutrient inventory and thereby the oceans susceptibility to develop anoxic conditions. In particular we find that besides sea level and sedimentation rate changes the temperature dependent decay of organic matter in the ocean is quite significant. Based on the sensitivity investigation we then explore how the DIP inventory may have changed through Earths history. For the Phanerozoic we couple the model to a carbon cycle model similar to GEOCARB III. The nutrient inventory of the world ocean and its base state oxygen concentration is thereby a function of the model derived weathering and temperature changes as well as global sedimentation rate. Even though the model derived oceanic nutrient and oxygen changes have a large uncertainty, the modeled changes possibly have implications for development of ocean anoxia during the Phanerozoic.
PP33C-06
Carbon Isotope Excursions, Chalk Seas, and Black Shales: A new Look at Oceanic Anoxic Event III
Organic carbon (OC) and carbonate burial fluxes to marine sediment are thought to have important effects on the dynamics of carbon cycling and global climate through the ocean-atmosphere-land linkage. Interpretations of such dynamics in the Cretaceous are supported by observations of widespread OC-rich strata and positive $\delta$$^{13}$C excursions in carbonates and OC of about 2$\permil$. These episodes are referred to as Oceanic Anoxic Events (OAEs). One of the more unusual OAEs in the Cretaceous (the Coniacian OAE III) is characterized by a smaller positive C-isotope excursion of 0.5$\permil$, limited spatial extent of OC-rich strata, and a protracted temporal extent of > 1 Ma. This event, the last of the Cretaceous OAEs, is unique in that it coincides with a major proliferation of coccolithophores and extensive deposition of Late Cretaceous chalks in shelf and epeiric settings. In order to evaluate the impact of widespread chalk deposition on C-isotopic excursions of this age, we developed a geochemical model of Late Cretaceous carbon cycling and $\delta$$^{13}$C changes of ocean water. Data from a range of pelagic and hemipelagic facies representing open ocean, epeiric, and shelf settings are used to constrain the model. Critical components to the model are the accumulation rates of OC and carbonate-C for each locality. These rates are more important than the simple fraction of C buried as OC, as bulk sedimentation rates vary with respect to both locality and age. The model input data include the molar accumulation rates of C as OC and CaCO$_{3}$ for each locality, where the burial rates are assumed balanced by input fluxes over geologically short intervals of time, so that alkalinity and DIC are conserved. The model results of $\delta$$^{13}$C$_{DIC}$, $\delta$$^{13}$C$_{carb}$ and $\delta$$^{13}$C$_{org}$ indicate, as expected, that deposition of nearly pure chalks drives $\delta$$^{13}$C$_{DIC}$ negative, whereas deposition of black shales drives it positive. The molar accumulation rates of OC and carbonate-C for chalk, marl, and black shale facies are used to calculate their relative contributions with respect to changes in $\delta$$^{13}$C$_{carb}$ of 0, +0.5, +2.0, and +2.8$\permil$. These model simulations quantify the proportional contributions of these different facies to the observed Cretaceous carbon-isotope excursions. The results indicate that the C-isotopic signatures of the Cretaceous oceans are driven by sedimentation rates and the relative proportions of OC and C-carbonate, both of which vary by sedimentary facies. Therefore, the areal distribution of facies is an important factor with respect to changes in the C-isotopic signature of the Cretaceous oceans. Thus widespread deposition of chalk facies in the Late Cretaceous likely muted positive $\delta$$^{13}$C shifts in ocean water, despite the OC enrichment in many deep ocean basins. Conceivably, the termination of the Cretaceous OAE series (Aptian to Cenomanian) may be linked to higher rates of carbonate bioproductivity and storage, resulting in only smaller positive shifts in $\delta$$^{13}$C of Late Cretaceous ocean water. The results also underscore the importance of accumulation rate calculations for various hemipelagic facies across the globe and, given the brevity of many OAEs, emphasize the need for high-resolution timescales.
PP33C-07
Rapid climate change and associated black shale deposition during past greenhouse conditions: The lower Albian Oceanic Anoxic Event 1b
The lower Albian Oceanic Anoxic Event 1b (OAE 1b) represents a distinct short lived perturbation (45 kyrs) of the global carbon cycle during Cretaceous greenhouse conditions. The event is recognized based on negative carbon isotope excursions recorded in carbonates, bulk organic matter and molecular marker components attributed to marine and terrestrial derived organic matter. We report data from DSDP Site 545 Mazagan Plateau and the Vocontian Basin of Southern France indicating a sudden increase in sea surface temperature by 3 degree Celsius within a few hundred years based on Tex 86 measurements. Associated is a distinct increase in marine productivity leading to black shale deposition and an elevated flux of continental derived nutrients to the Atlantic and Tethys Ocean documented by inorganic geochemical proxies. Our data are in good agreement with a period characterized by an enhanced hydrological in tropical and subtropical regions during the event. Results from box model experiments show that the distinct geochemical signature of OAE 1b is best explained by the release of methane from gas hydrates. Based on the modelling results the release of approximately one tens of the amount of gas hydrates known today is required to explain the observed carbon isotope excursions.
PP33C-08
A sulfate starved ocean and the perturbation of the global carbon and sulfur cycles in the Early Cretaceous
Recent high resolution $\delta^{13}$C and $\delta^{34}$S data show that the onset of the Aptian $\delta^{13}$C and $\delta^{34}$S excursions are contemporaneous and inversely correlated. This negative correlation is difficult to explain if we assume a modern ocean chemistry. However, new data suggest that during most of the last 550~Ma, the ocean had much lower sulfate concentrations than today. Here we show that the evaporites deposited during the opening of the South Atlantic removed enough sulfate from the ocean to decrease microbial sulfate reduction rates in the sedimentary column, resulting in a ~50% decrease of pyrite burial rates. As microbial sulfate reduction is also the major pathway for organic matter oxidation in the sedimentary system, this also decreased organic matter remineralization rates, causing the observed inverse relationship between the burial flux of organic matter and pyrite observed in their respective isotope signatures. Analyzing the Early Cretaceous perturbations of the global carbon and sulfur cycles as a response of the microbial biosphere to transient changes in seawater chemistry provides an elegant explanation of the existing data, without the need for dramatic changes in volcanic fluxes or the oceanic alkalinity budget.