PP51B-1333 0800h
The Late Miocene Carbon Isotope Shift and Marine Biological Productivity.
The late Miocene global carbon isotope shift of approximately 1 per mil is not well understood. Is it linked to ocean-related processes such as the Biologic Bloom \(Farrell et al., 1995\), or to changes in type \(C3/C4 plants\) or cover of terrestrial vegetation? Here we examine the evolution of marine biological productivity during the isotope shift at ODP Site 846 \(Pacific equatorial upwelling, where the Biologic Bloom has been first described, Farrell al, 1995\) and at Indian Ocean Site 721 \(monsoon-driven upwelling\), and compare their productivity history with non upwelling locations in the Atlantic Ocean. The onset of the carbon isotope shift is accompanied at all locations by an increase in paleoproductivity derived from benthic foraminiferal accumulation rates \(expressed as gC/cm2 * ky; Huerguera, 2000\) and increased abundance of Uvigerina spp.. At the equatorial upwelling sites the increase is comparable to half present-day values to present-day values; in the Atlantic Ocean paleoproductivity increases from present-day up to 3 times present-day values. But the productivity maxima are not concurrent. The carbon isotope shift is accompanied by severe carbonate dissolution and reduced ventilation of bottom waters, as reflected in the occurrence of pyrite and good preservation of cartilageous fish debris. Carbonate preservation is good since about 6 Ma despite high productivity. We discuss changing deep water circulation patterns, increased weathering and continental nutrient delivery, as well as erosion of terrestrial vegetation as possible factors to explain our findings.
PP51B-1334 0800h
Stomatal Frequency and Atmospheric CO2: a Model Based on Photosynthesis and Gaseous Diffusion
Gaseous exchange in land plants occurs via micropores on the plant surface, the stomata. The stomatal frequency (SF) of leaves has attracted considerable interest with respect to increasing atmospheric CO2 since Woodward (1987) demonstrated that this parameter changes inversely with CO2 in various species. The response is due to 1) individual phenotypic plasticity and 2) evolutionary change, depending on the considered time scale. The SF-CO2 response is regarded to represent a valuable device for determining past atmospheric CO2 concentration and SF data from fossil plant material are therefore often used as CO2 proxies. There are, however, numerous difficulties which have to be considered, such as: 1) high variance of the data, especially for fossil material, 2) interspecific differences of the response, 3) the CO2 ceiling (= weak or no response under CO2 concentration higher than ambient) and 4) differences between short-term and long-term responses. Although processes related to plant gaseous exchange are assumed to represent the causal basis for the response, no clear explanatory model has yet been proposed and even doubts have emerged about the real existence of this phenomenon. In this contribution it is shown that results obtained with a model based on diffusion and assimilation suggest that the SF-CO2 response is a structural adjustment of maximum stomatal conductance. The results 1) offer a clear explanation for the often observed weak response of stomatal frequency to CO2 levels higher than ambient, 2) provide a simple equation for calculating ancient CO2 levels from stomatal data, and 3) can contribute to predictions concerning plant reactions to elevated CO2 levels in the future.
PP51B-1335 0800h
Cretaceous records of atmospheric CO$_{2}$ from fossil conifers
Experimental studies indicate the stomatal characteristics of conifer shoots, like angiosperm leaves, are sensitive to changes in the global atmospheric CO$_{2}$ concentration. Consequently, fossil conifers may serve as valuable archives of pre-Quaternary paleo-CO$_{2}$ information, complementing that of other proxies and geochemical models of the long-term carbon cycle. Our study investigated this possibility by focussing on the fossil genus {\it Frenelopsis}, which has robust, degradation resistant cuticles and a continuous stratigraphic record throughout most of the Cretaceous. We measured the stomatal index (proportion of epidermal cells that are stomata) of {\it Frenelopsis} cuticles from eight Western European localities dating from Late Barremian to Late Cenomanian (131 to 90 Ma). Calibrating the record using nearest-living equivalent species indicated atmospheric CO$_{2}$ levels of 1000-1200 p.p.m.v. during the Barremian/Aptian and lower values of 500-1000 p.p.m.v. during the Albian and Cenomanian. The results are in general agreement with earlier CO$_{2}$ estimates from paleosols and phytoplankton and suggest quite large variations in atmospheric CO$_{2}$ took place during the Cretaceous, some of which may be linked with short-lived cooling events revealed by high resolution oxygen isotope studies of marine sediments.
PP51B-1336 0800h
Radiolarian and Sedimentologic Paleoproductivity Proxy Record From the Benguela Upwelling System, DSDP Site 532, 0-6 Ma
The Neogene history of the Benguela upwelling system is fundamental to better understand both regional climate change and how the global ocean cycles nutrients, including carbon. We examined the 0-6 my record of the system at ca. 400 kyr spacing from Walvis Ridge DSDP Leg 75 Site 532. More than 70 radiolarian taxa were counted and two indices computed: a water depth ratio index reflecting mid water export productivity, and the URI index of species that are biogeographically restricted to tropical upwelling regions. Both proxies have been applied so far only to the late Pleistocene. Other proxies measured on the samples include BFAR, TOC, % and MAR carbonate, bulk opal and radiolarian presevation (semiquantitative), foraminiferal dissolution indices, and stable isotopes of oxygen and carbon. The radiolarian water depth index, and to a lesser degree the URI, show gradually increasing productivity in the early Pliocene and high productivity through the late Pleistocene. This general productivity pattern for the region has been established by numerous prior studies. The radiolarian water depth index is strongly correlated to TOC, and importantly is not affected by low opal concentration and poor radiolarian preservation in the basal Pliocene and late Pleistocene. Carbonate, and carbonate microfossil based indices such as BFAR by contrast show largely inverse trends to radiolarians and TOC, but are interpreted as being predominantly dissolution controlled, as indicated by the carbonate dissolution proxies. Our results show that radiolarian faunas yield useful productivity proxies in these upwelling sediments as far back as the latest Miocene. Further, radiolarian based productivity proxies are relatively robust to preservation or the supply of silica to surface waters, which is thought to affect bulk opal based measures of productivity in the region.
PP51B-1337 0800h
Emergence of the Namib Desert During the Neogene, and Correlations With Global Palaeoclimate Changes
The Namib is one of the oldest known desert systems, hence its formation and maintenance are of considerable regional and global importance. Fluctuations in Atlantic climate and oceanic circulation patterns directly affect the ecology of the Namibian desert, and the entire South African west coast area, on a variety of timescales. The discovery of mammalian fossils and ratite eggshells has provided a biochronology, while the ratite eggshells have allowed stable oxygen and carbon isotope studies of this material to pinpoint the plant dietary base of these birds and to provide clues about changing environments through time. The results of the existing multidisciplinary study provide a sequence showing that during the lower Miocene, prior to 17 Ma, (fluviatile environments, terraces of the Proto-Orange river) a tropical/subtropical climate existed. The onset of aridification is correlated with appearance of the first aeolianites in the basal middle Miocene period. In the Southern Namib, cross-stratification in aeolianites represent the lower slipface of crescent dunes. The predominant northeast-northwest azimuths of the cross-bedded strata indicate deposition under a prevailing southerly wind regime. This regime remained in place throughout the Neogene. The aeolianites of the Northern Namib present different lithologies from those in the south: sand sheet deposits rich in cemented roots (stabilization phase of the arid system), cross-stratification of complex dunes and deflation surfaces related to water flowing from nearby inselbergs. The azimuths of these cross-bedded strata indicate deposition under westerly/southerly winds and also easterly and northeasterly winds during the Late Miocene (at present the bergwinds produce similar conditions). Isotopic data from the ratite eggshells shows advance of C4 grasses in this region at the end of the Miocene, and emergent differences in the plant cover of the southern versus northern sectors. Eggshell carbon isotope composition during the C3- period (prior to the spread of any C4 grasses) tracks isotopic variability of C3 plants relative to recently estimated low global atmospheric CO2 concentrations and pluviometry. Comparisons between continental Namib data with oceanic data (oxygen isotopes in benthic foraminifera, and the sea level curve) provide a broader global framework for the history of this desert. Two aridification periods appear to have occurred: (i) during the Middle Miocene (contemporaneous with establishment of the Antarctic ice-cap), and (ii) intensification of aridity during the Lower Pliocene (related to growth of the Arctic Ice Cap and upwelling of the Benguela cold water cells).
PP51B-1338 0800h
Marine Plankton Revolution and the Closure of the Circum-Tropical Seaways
Climatic and tectonic events in the Neogene had a profound effect on marine pelagic environments. With the cooling in the high latitudes, latitudinal and vertical temperature gradients in the ocean increased. Additionally, the throughflow through the major tropical seaways became increasingly restricted, limiting the exchange of planktic organisms. We have analysed the size record of Neogene foraminifers and compared it with existing data on size in calcareous nannoplankton. Our data show that dramatic size increase in planktic foraminifers was not unidirectional, but showed size reductions during the Late Miocene carbonate crash, the biogenic bloom event and the Pliocene climate optimum. From 4.1 Ma onwards, unprecedented large sizes are recorded in the oligotrophic tropical settings, which do not occur in the temperate or upwelling settings. We relate the size trend to a closure of the circum-tropical seaways. By 4.2 Ma, an effective closure of the Isthmus appears to be followed by the evolution of foraminifers of an unprecedented large size. The changes in size do not reflect evolution of new, larger species, but they affect existing lineages, e.g. {\it G. conoidea - G. inflata} lineage or the {\it G. plesiotumida - tumida} lineage. The closure of the tropical seaways is a novel tectonic constellation in the evolutionary history of planktic foraminifera. The closure has strengthened the meridional heat transport, enhanced stratification, changed surface water density, and thermocline depth. This will have increased the number of available niches in the upper water column. Enhanced adaptation to the new niches in the tropical oceans might have allowed for larger sizes in assemblages, but have not lead to an increased number of species, as shown by the decoupling of size and diversity in the Plio-Pleistocene. Increased stratification restricts the transport of nutrients to the surface and growth to large size in coccoliths, but promoting foraminiferal growth. This decoupling of size and diversity and the divergence of mid- and low latitude size evolution in foraminifers, in contrast to the reduction in size in coccolithophorids , point to a revolutionary change in the niches in the upper water column during the Neogene caused by the unique closure of the circum-tropical seaways.
PP51B-1339 0800h
In-situ boron isotopic analysis of modern and ancient marine carbonates by LA ICP-MS
The boron isotopic composition of marine carbonates has been shown to be useful as a tracer of paleo-pH, and for the reconstruction of ancient pCO$_{2}$ through time. Conventional wet chemical and TIMS techniques for the separation and measurement of $\delta$$^{11}$B are hampered by complicated, tricky chemistry and boron contamination. Based on an {\it in-situ}, laser ablation inductively coupled plasma (LA ICP-MS) analysis of boron isotope compositions in MORB glasses (LeRoux et al., 2003), we have modified the technique to allow for the analysis of synthetic and natural carbonates with $ \pm$1$\permil$ (2$\sigma$) precision. A synthetic carbonate was prepared using nominally B-free calcite powder which was doped with a 1000 ppm NIST SRM 951 solution and fused in a Pt capsule at 1500$^{o}$C and $>$ 7 kbar for 10 min in a piston-cylinder apparatus. The resulting synthetic carbonate contained about 30 ppm boron. Analyses were done at the Department of Terrestrial Magnetism, Carnegie Institution of Washington with a CETAC LSX-200 Nd-YAG frequency quadrupled (266 nm) laser connected to a VG Axiom multi-collector magnetic sector ICP-MS. This instrument is equipped with a three electron multiplier detector which allows the simultaneous collection $^{10}$B and $^{11}$B in pulse counting mode. The technique interspersed repeat measurements of NIST SRM glasses (610 and 612 with independently measured compositions equivalent to SRM 951) with unknowns. The 951-doped synthetic carbonate gave a $\delta$$^{11}$B = 0.50 $\pm$ 1.08$\permil$ (n = 24) or within analytical uncertainty of pure SRM 951. A modern coral carbonate ({\it Porites sp.}) was similarly analyzed and gave a $\delta$$^{11}$B value of 26 $\pm$ 0.38$\permil$ (n = 3), which corresponds to the ocean pH of ~8.2 as predicted from seawater composition. Future analyses of a variety of coral species from Bermuda and the Florida Keys, collected at depths between 6 and 27 meters will allow the evaluation of species type, growth effects, and water depth on boron isotopic compositions. Ancient coral samples formed at intermediate to low sea levels from the Huon Gulf in the Pacific Ocean and from intermediate and high sea levels from Barbados in the Atlantic Ocean, which have been dated by U series disequilibria techniques to have grown during the last several glacial/interglacial cycles, will be studied to understand changes in oceanic pH associated with orbital cycles and other climatic perturbations.
PP51B-1340 0800h
Top Down and Bottom up: A Comparison of Coccolith Sr/Ca Ratios and Benthic Foraminiferal Accumulation Rates as Indicators of Paleoproductivity
Recent studies have linked coccolith Sr/Ca ratios to the growth rate of the coccolithophorid algae (Stoll and Schrag, 2000). While this link implies a correlation between coccolith Sr/Ca ratios and primary productivity, application of this proxy has not yet been fully tested against other paleoproductivity indicators. Here we compare down-core measurements of coccolith Sr/Ca ratios to down-core changes in benthic foraminiferal accumulation rates (BFAR), a proxy for export production, from two Ocean Drilling Program sites in the Atlantic (western tropical Atlantic Site 925 and subpolar North Atlantic Site 982). The sediments used for this analysis span the late Miocene through early Pliocene (9-3 Ma), which was chosen for its significance as a period marked by a massive "biogenic bloom". Although not synchronous, increases in paleoproductivity are apparent in both BFAR records (Diester-Haass et al., submitted). Our preliminary Sr/Ca record from Site 982, which is based on a single coccolith size fraction, shows that the overall long-term trend is matched and that there are a number of synchronous maxima and minima. Statistical analysis confirms a significant correlation between coccolith Sr/Ca ratios and BFAR at this site. Our preliminary interpretations demonstrate that a relationship exists between coccolith Sr/Ca and BFAR, supporting our understanding of the proxies as paleoproductivity indicators. Further work includes a separation of discrete coccolith size fractions and down-core monitoring of species assemblages to better constrain the potential effects of nannofossil species assemblage changes on coccolith Sr/Ca ratios.
PP51B-1341 0800h
Marine Barite and Carbonate Accumulation Rates From Sediment Cores Spanning the Eocene-Oligocene Boundary
The Eocene-Oligocene (E-O) boundary marked a period of significant worldwide oceanographic and climatic changes. The opening of the Drake Passage around that time, and the resulting formation of a circum-Antarctic sea, allowed for thermal isolation and glaciation on the Antarctic continent. Coupled with this were several global decreases in temperature and a drop in atmospheric CO2. Since export productivity is considered one of the major driving factors of the global carbon cycle, understanding the changes in ocean productivity is important to understanding the mechanism for the climate changes across the E-O boundary. Marine barite is associated with export production to the deep ocean, and barite accumulation rates can be used as a proxy of productivity. Carbonate accumulation rates can be used to assess the productivity of calcite secreting organisms as modified by calcite dissolution. Here we present barite and carbonate accumulation rates for several cores from the Shatsky Rise in the Pacific Ocean that span the E-O boundary. The goal of this data is to assess changes in ocean productivity, seawater chemistry, and changes in the CCD during this time.
PP51B-1342 0800h
Climate Sensitivity to Vegetation Distribution in the Early Eocene
There are large uncertainties associated with the reconstructions of global vegetation distributions for past time periods in Earth history. In order to investigate the influence of the global distribution of vegetation on modeled early Eocene climate, we carried out two experiments with the National Center for Atmospheric Research Community Atmosphere Model (v.2.0.2.). Both experiments used an atmospheric CO2 level of 560 ppm and sea surface temperatures generated in a previous Eocene modeling study (Huber and Sloan, 2001). The experiments differed only in their prescribed vegetation distributions. The first experiment used output from an Eocene study with a dynamic global vegetation model (DGVM) (Shellito and Sloan, in preparation), while the second case used a vegetation scheme based on fossil flora (Sewall et al., 2000). In creating the Sewall et al. (2000) vegetation scheme, no specific atmospheric CO2 level was specified, whereas the DGVM in the study by Shellito and Sloan (in preparation) allowed the initial vegetation to reach equilibrium with a pCO2 level of 560 ppm. Two additional experiments were run with higher atmospheric CO2 concentrations of 1120 ppm; one used output from an Eocene DGVM study with a pCO2 of 1120 ppm (Shellito and Sloan, in preparation), while the second case used the Sewall et al. (2000) vegetation. Comparison of early Eocene climates resulting from each pair of pCO2 experiments allows us to assess the influence of prescribed vegetation upon climate. By evaluating these results at two different levels of atmospheric CO2, we can investigate the combined effects of pCO2 and vegetation upon the resulting early Eocene climate.