American Geophysical Union (AGU)
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Carbon isotope ratio of Cenozoic CO<sub>2</sub>: A comparative evaluation of available geochemical proxies
American Geophysical Union (AGU) - Tập 25 Số 3 - 2010
Deglacial whole-ocean δ<sup>13</sup>C change estimated from 480 benthic foraminiferal records
American Geophysical Union (AGU) - Tập 29 Số 6 - Trang 549-563 - 2014
Fluctuations in the trophic resource continuum: A factor in global diversity cycles? The trophic resource continuum (TRC) in euphotic zones of the world's oceans is the spectrum of conditions from the richest runoff and upwelling areas to the most nutrient deficient subtropical seas. That spectral range has likely varied through geologic history as more efficient phytoplankton taxa evolved, as oceanic circulation rates changed, and in response to oceanic‐mixing events. Reduced rates of oceanic turnover should produce regionally intensified eutrophy and oligotrophy, that is, TRC expansion. Because oceanic waters as a whole are relatively nutrient rich, increased rates of oceanic turnover should prevent extremely eutrophic and extremely oligotrophic conditions from developing, causing TRC contraction. Global biotic diversities in the oceans are strongly influenced by communities in oligotrophic environments. Nutrient deficient waters are more stable and more transparent than richer waters, permitting higher degrees of specialization. Sparse nutrient supplies necessitate smaller phytoplankton and longer, more complex food chains. Preservation potential of oligotrophic biotas is also greater because calcareous taxa are more prevalent and because calcium carbonate is less apt to be bioeroded or dissolved. Fluctuations in the TRC should profoundly affect diversity of habitats within the euphotic zone of the oceans. The evolutionary response to a TRC expansion, diversification, would occur in a geologic time frame of millions of years, while evolutionary responses to a TRC contraction, including extinction, could occur in a biological time frame of generations. A simple model indicates that a reduction in the trophic resource continuum, from expansion during a major marine transgression, down to modern levels could result in loss of 40% of the euphotic habitats. How fluctuations in the TRC control diversity of euphotic habitats in the oceans provides an hypothesis by which to interpret global taxonomic diversity cycles, as well as taxonomically selective extinctions, changes in species dominance, and some morphologic trends.
American Geophysical Union (AGU) - Tập 2 Số 5 - Trang 457-471 - 1987
How synchronous are neogene marine plankton events? An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGU's FTP account using ANONYMOUS as the username andGUEST as the password. Go to the right directory by typing CDAPEND. Type LS to see what files are available. Type GET and thename of the file to get it. Finally, type EXIT to leave the system.)(Paper 94PA01456, How synchronous are Neogene marine planktonevents?, by C. Spencer‐Cervato, H. R. Thierstein, D. B. Lazarus, andJ‐P Beckmann). Diskette may be ordered from American GeophysicalUnion, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00.Payment must accompany order. We analyzed the synchrony and diachrony of commonly used Neogene biostratigraphic events from data published in the Initial Reports of the Deep Sea Drilling Project (DSDP) and in the Proceedings of the Ocean Drilling Program (ODP). On the basis of the combined biostratigraphic and magnetostratigraphic evidence, new Neogene age models were constructed for 35 globally distributed DSDP and ODP holes. Biostratigraphic events from the four major plankton groups (calcareous nannofossils, diatoms, planktonic foraminifera, and radiolarians) were compiled from DSDP and ODP reports. After the elimination of possible sources of error such as stratigraphic hiatuses and reworking of specimens, 124 biostratigraphic events that occurred in at least four holes were analyzed in detail: for each event a biochronologic age estimate was derived by projection of the depth of the event onto the line of correlation of each hole, and from these a global mean age for each event was calculated, together with its standard deviation. Average standard deviations for event ages by fossil group are: calcareous nannofossil first appearance datums (FADs): 0.57 m.y. (21 events), calcareous nannofossil last appearance datums (LADs): 0.60 m.y. (25 events), diatom FADs: 0.57 m.y. (7 events), diatom LADs: 0.85 m.y. (14 events), planktonic foraminifera FADs: 0.88 m.y. (22 events), foraminifera LADs: 0.68 m.y. (16 events), radiolarian FADs: 0.30 m.y. (9 events), radiolarian LADs: 0.31 m.y. (10 events). Since the average sample spacing in the sites used for this analysis is only 0.185 m.y., we have examined the data for true patterns of diachrony and for other biases. Diachrony is more frequent among cosmopolitan than among endemic taxa, thus there is a general trade‐off between the obtainable age precision and the geographic extent of a bioevent. Precision of age calibrations also decreases with increasing age. Although some of these features may be due to investigator bias, they appear in part to be real phenomena, and thus could also provide opportunities for further exploration of important paleobiological processes, such as change in environmental gradients through time, evolutionary adaptation of species populations and migration due to water mass changes.
American Geophysical Union (AGU) - Tập 9 Số 5 - Trang 739-763 - 1994
Benthic phosphorus regeneration, net primary production, and ocean anoxia: A model of the coupled marine biogeochemical cycles of carbon and phosphorus We examine the relationships between ocean ventilation, primary production, water column anoxia, and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon (C) and phosphorus (P). The elemental cycles are coupled via the Redfield C/P ratio of marine phytoplankton and the C/P ratio of organic matter preserved in marine sediments. The model assumes that on geologic timescales, net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone. The model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water‐sediment interface. Evidence from marine and lacustrine settings, modern and ancient, demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia‐dysoxia expands. Steady state simulations show that a reduction in the rate of thermohaline circulation, or a decrease of the oxygen content of downwelling water masses, intensifies water column anoxia‐dysoxia and at the same time increases surface water productivity. The first effect reflects the declining supply of oxygen to the deeper parts of the ocean. The second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides. Sedimentary burial of organic carbon and authigenic calcium phosphate mineral (francolite), on the other hand, is promoted by reduced ocean ventilation. According to the model, global‐scale anoxia‐dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system. Consequently, higher rates of primary production and organic carbon burial can be achieved, even when the continental supply of reactive phosphorus to the oceans remains unchanged.
American Geophysical Union (AGU) - Tập 9 Số 5 - Trang 677-692 - 1994
An apparent contradiction in the role of phosphorus in Cenozoic chemical mass balances for the World Ocean Little is known about the fluxes to and from the ocean during the Cenozoic of phosphorus (P), a limiting nutrient for oceanic primary productivity and organic carbon burial on geologic timescales. Previous studies have concluded that dissolved river fluxes increased worldwide during the Cenozoic and that organic carbon burial decreased relative to calcium carbonate burial and perhaps in absolute terms as well. To examine the apparent contradiction between increased river fluxes of P (assuming P fluxes behave like the others) expected to drive increased organic carbon burial and observations indicating decreased organic carbon burial, we determined P accumulation rates for equatorial Pacific sediments from Ocean Drilling Program leg 138 sites in the eastern equatorial Pacific and leg 130 sites on the Ontong Java Plateau in the western equatorial Pacific. Although there are site specific and depth dependent effects on P accumulation rates, there are important features common to the records at all sites. P accumulation rates declined from 50 to 20 Ma, showed some variability from 20 to 10 Ma, and had a substantial peak from 9 to 3 Ma centered at 5–6 Ma. These changes in P accumulation rates for the equatorial Pacific are equivalent to substantial changes in the P mass balance. However, the pattern resembles neither that of weathering flux indicators (87 Sr/86 Sr and Ge/Si ratios) nor that of the carbon isotope record reflecting changes in organic carbon burial rates. Although these P accumulation rate patterns need confirmation from other regions with sediment burial significant in global mass balances (e.g., the North Pacific and Southern Ocean), it appears that P weathering inputs to the ocean are decoupled from those of other elements and that further exploration is needed of the relationship between P burial and net organic carbon burial.
American Geophysical Union (AGU) - Tập 9 Số 4 - Trang 513-527 - 1994
Tethyan carbonate carbon isotope stratigraphy across the Jurassic‐Cretaceous boundary: An indicator of decelerated global carbon cycling? The carbon isotope record in four pelagic carbonate sections from the Southern Alps (northern Italy) across the Jurassic‐Cretaceous boundary has been correlated to biostratigraphy and magnetostratigraphy. The carbon isotope curve from bulk carbonates shows a decrease from Kimmeridgian to Early Tithonian (CM24–CM22) values of δ13 C=+2.07 (± 0.14)‰ to Late Tithonian and Berriasian (CM18–CM14) values of δ13 C=+1.26 (± 0.16)‰. The change in the carbon isotope record coincides with changes in Tethyan calcite and silica accumulation rates, with a drop in the calcite compensation depth in the Atlantic and Tethys oceans and with changes in organic carbon burial along the Eurasian margin of the Tethys. Reduced surface water productivity due to diminished transfer rates of biolimiting elements into the Atlantic and Tethys oceans can explain these observations. The decreased transfer rates of elements such as silica or phosphorus from continents into the oceans resulted from drier climatic conditions and decreased water runoff on continents bordering the Tethys and Atlantic oceans. The proposed changes in Tithonian ‐ Berriasian ocean chemistry and paleoclimate suggest that variations in the global carbon cycle were coupled with changes in the global hydrological cycle and in associated material cycles.
American Geophysical Union (AGU) - Tập 4 Số 4 - Trang 483-494 - 1989
Sedimentation history of the South Pacific pelagic clay province over the last 85 million years Inferred from the geochemistry of Deep Sea Drilling Project Hole 596 Geochemical analyses of sediments from the top 24.5 m of Deep Sea Drilling Project hole 596 (23°51.20′S, 169°39.27′W) show great variability in the composition of pelagic clays accumulated in the South Pacific since the late Cretaceous. Elemental associations indicate that most of this variability can be attributed to variations in abundances of six sediment end‐member components: detrital (eolian), andesitic (volcanic), hydrothermal, hydrogenous, phosphate (fish debris), and biogenic silica. We develop a sedimentation model which is used to infer processes that might have influenced the accumulation rates of these components over the last 85 million years. The accumulation of eolian detritus in the South Pacific shows some similarities to that observed in the North Pacific and has been largely controlled by global climate trends in the Cenozoic. Much of the variation in the accumulation of other sediment components likely reflects the paleoceanographic evolution of the South Pacific. The most notable change in the sedimentary environment occurred at about the Paleogene/Neogene boundary. At that time, significant changes in the color, mineralogy, and chemistry of the sediment probably reflect major shifts in climate mode as well as oceanic circulation in the central South Pacific region.
American Geophysical Union (AGU) - Tập 7 Số 4 - Trang 441-465 - 1992
The Himalayas, organic carbon burial, and climate in the Miocene Cooling ages of rock in the Himalayas imply that rapid exhumation between the Main Central thrust system and the South Tibetan detachment system occurred between 21 and 17 Ma. The generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium, carbon, phosphorus, and other chemical weathering products to the ocean (Richter et al., 1992). The increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the Monterey formation and other deposits from this interval. A downdraw of atmospheric CO2 associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the Antarctic ice sheet by 15 Ma. The above scenario differs from the “Monterey hypothesis” of Vincent and Berger in that CO2 downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling. A carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 Ma predicts that absolute organic carbon burial increased over this interval while, at the same time, the fraction of carbon buried as organic matter versus carbonate decreased. This implies that the organic carbon cycle has acted as a net source of CO2 to the atmosphere over the late Cenozoic.
American Geophysical Union (AGU) - Tập 9 Số 3 - Trang 399-404 - 1994
Pacific pelagic phosphorus accumulation during the last 10 M.Y. As a limiting nutrient to marine life, phosphorus (P) is an effective tracer of today's marine productivity. The distribution of P in marine sediments likewise tracks the history of marine productivity because of its relative insolubility in seawater. CaCO3 , biogenic opal, terrigenous sediment, and total P have been measured in cores from nine Pacific sites (Deep Sea Drilling Project (DSDP) 65, 66, 310, 77, 62, 572, 463, 586, and GPC‐3) and one subantarctic (DSDP 266) site. These sites were specifically chosen to provide information on biota burial flux changes with time for sedimentary sinks that represent key oceanographic variables, i.e., rate of upwelling, water depth, and carbonate dissolution gradient. The accumulation rates of these components for the last 10 Ma were then calculated from determined core age versus depth plots, core bulk density, and porosity data. The accumulation of P weakly correlates with that of CaCO3 , moderately with that of total sediment, and very strongly with carbonate‐free accumulation. Two prominent peaks for all components occur at 2–3 Ma and 5–6 Ma, and record the chemical loading of dissolved CaCO3 , SiO2 , and P from glacially emergent continental shelves. These results indicate that continental shelf phosphorites form during interglacially high sea levels and correspond to low deep‐sea P accumulation rates, whereas glacially lowered sea levels allow for shelf bypassing and greater deep‐sea P accumulation rates.
American Geophysical Union (AGU) - Tập 3 Số 1 - Trang 113-136 - 1988
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