PP23C-01
An Alternative Mechanism for Producing Global Changes in the Carbon Isotopic Composition of Platform Derived Sediments
The stable carbon isotopic composition of the skeletons of carbonate organisms and sediments has been widely used not only in order to derive information upon the burial of organic material throughout the geological record, but also for correlative purposes. While the preferred type of material for analysis in sediments younger than ~ 200 Ma are microfossils derived from oceanic sediments, in older deposits one is forced to use the δ13C of bulk sediments deposited on and surrounding carbonate platforms. Such sediments are often problematic as a result of diagenesis and the fact that they are derived from a variety of different sources which have differing δ13C values. This can lead to the correlation of the δ13C in age equivalent strata prograding from a carbonate platform. A notable example of this is shown in the Bahamas which show good correlations between individual sequences, but little correlation to global carbon curves derived from planktonic and benthonic foraminifera. In order to assess whether similar phenomenon are present surrounding other carbonate edifices over the same time period, the δ13C was measured in Holocene to Miocene aged material from cores taken off the Maldives (Ocean Drilling Program Site 716), the Queensland Plateau (Site 817), the Great Barrier Reef (Sites 821 and 822), and the Great Australian Bight (Site 1126). All these sites showed similar patterns in the δ13C values of the bulk sediments compared to the Bahamas between the present day and the middle Miocene and no correlation with the global δ13C curve. It is suggested that the synchronous variations in the δ13C values in sediments of equivalent age are related to variations in sea level which flood the platform, allowing the production of carbonate sediments with positive δ13C values. These sediments become admixed into pelagic sediments producing an apparent global signal. These results have obvious implications on the use of δ13C records in carbonate derived sediments to interpret the paleocarbon cycle in that widespread changes in the δ13C cannot be used as evidence that variations are related to the burial and oxidation of organic material.
PP23C-02
Quasi-100 ky glacial-interglacial cycles triggered by subglacial burial carbon release
A mechanism is proposed in which climate, carbon cycle and icesheets interact with each other to produce a feedback that can lead to quasi-100 ky glacial-interglacial cycles. A central process is the burial and preservation of organic carbon by icesheets which contributes to the observed glacial-interglacial CO2 change (the glacial burial hypothesis, Zeng, 2003). Allowing carbon cycle to interact with physical climate, here I further hypothesize that deglaciation can be triggered by the ejection of glacial burial carbon when a major icesheet grows to sufficiently large size after a prolonged glaciation so that subglacial transport becomes significant. Glacial inception may be initiated by CO2 drawdown due to a relaxation from a highbut transient interglacial CO2 value as the land-originated CO2 invades into deep ocean via thermohaline circulation and CaCO3 compensation. Also important for glacial inception may be the CO2 uptake by vegetation and soil regrowth in the previously ice- covered regions. When tested in a fully coupled Earth system model with comprehensive carbon cycle components and semiempirical physical climate components, it produced under certain parameter regimes self- sustaining glacial-interglacial cycles with durations of 93 ky, CO2 changes of 90 ppmv, temperature changes of 6C. Since the 100 ky cycles can not be easily explained by the Milankovitch astronomical forcing alone, this carbon-climate-icesheet mechanism provides a strong feedback that could interact with external forcings to produce the major observed Quaternary climatic variations. It is speculated that some glacial terminations may be triggered by this internal feedback while others by orbital forcing. Some observable consequences are highlighted that may support or falsify the theory. http://www.atmos.umd.edu/~zeng
PP23C-03
Preliminary Results From The CHEETA Cruise Transect: Bulk And Molecular Proxies For Terrigenous Organic Matter Supply
In July, 2007 a sediment coring and water sampling cruise aboard R/V Oceanus was conducted off the Portuguese and NW African margins as part of the CHEETA (Changing Holocene Environments of the Eastern Tropical Atlantic) program. Gravity core and multicore pairs were recovered at 28 stations along a transect from 40°N-15°N paralleling the continental slope. One of the major goals of the CHEETA program is to utilize multiple organic proxies including the abundance and isotopic composition of higher plant leaf wax alkanes, and the distribution of soil-specific bacterial tetraether and hopanoid lipids to constrain regional variations in continental supply. The meridional array of multicore top samples cover different climate and vegetation zones in the SW Mediterranean and NW Africa, that transmit terrigenous signals to the adjacent continental margin via both eolian and fluvial processes, providing a rare opportunity to assess the sensitivity of these different proxies to continental supply, climate, and vegetation under modern conditions. The molecular analyses are used to assess the validity of different continental proxies, in particular of soil organic matter, for reconstruction of past continental climate and hydrological conditions in subtropical and tropical eastern Africa. Index terms 4912 Biogeochemical cycles, processes, and modelling 1631 Land/atmosphere interactions
PP23C-04
Cerium Anomalies in Fossil Fish Teeth Reveal Changes in Bottom Water Oxygenation
Shale-normalized rare earths and yttrium (REY) concentrations of fossil fish teeth in deep sea sediments display prominent negative cerium (Ce) anomalies and positive yttrium (Y) anomalies. These features are ultimately inherited from seawater and strongly indicate that fossil fish teeth preserve a seawater REY signature. In seawater, Ce+3 is oxidized to Ce+4, and Ce becomes depleted relative to the other REY's as it partitions into other phases (e.g., ferromanganese oxyhydroxides). The magnitude of Ce depletion in a water mass is thus related to its oxygen content. We hypothesize that changes in the oxygenation of bottom waters may be revealed by examining downcore variability in the magnitude of the Ce anomaly of fossil fish teeth. To test this hypothesis, REY concentrations were measured on samples of cleaned fossil fish teeth recovered from the late Paleogene to early Eocene sections of Ocean Drilling Program (ODP) Sites 1262 and 1263 (lower and upper Walvis Ridge, ODP Leg 208, South Atlantic Ocean). These sites are vertically offset (early Eocene paleodepths were 3700 and 1700 meters, respectively) and have been extensively studied to characterize the oceanic response to the Paleocene/Eocene Thermal Maximum (PETM). Manganese enrichment factors (Mn EF) determined from total digestions of samples from these sites reveal abrupt changes in the oxygenation of bottom waters across the PETM interval. Mn EF's decrease to crustal values (~1) during the PETM, which reflects the reduction of Mn oxides as bottom water oxygen levels were depleted. Mn EF's begin to increase to 6-8 during the ‘recovery phase' following the PETM. The Ce anomaly for these samples, Ce/Ce*, was calculated according to the geometric approach reported by Lawrence et al. (2006, Aquatic Geochemistry 12, 39-72), where Ce* represents an interpolation of the expected shale-normalized Ce concentration from near neighbors. In this notation, when Ce/Ce* = 1 no Ce anomaly is present. At upper Walvis Ridge downcore Ce anomalies show a distinct excursion from pre-PETM values between 0.7-0.8 to values ~1.0 during the PETM event. The values return to pre-excursion levels during the recovery phase following the PETM. The results of this study closely follow the evolution of Mn EF's at Walvis Ridge sites. The smallest Ce anomalies (i.e., Ce/Ce* = 1) coincide with Mn EF's of 1, which provide independent evidence for low oxygen levels in the bottom waters bathing these locations. These results confirm the hypothesis that Ce anomalies in fossil fish teeth can be used to assess changes in the oxygenation of bottom waters.
PP23C-05
Late Cretaceous Evolution of North Atlantic Deep Waters: Neodymium Isotopic Evidence From the Cape Verde Basin (DSDP Leg 41, Site 367) and Bermuda Rise (DSDP Leg 43, Site 386)
The εNd values of Late Cretaceous fish teeth and fish debris have been determined for two deep- ocean sites in the North Atlantic: DSDP Site 367 in the Cape Verde Basin and DSDP Site 386 on Bermuda Rise. These samples provide the first Cretaceous εNd data for sediments deposited at depths of modern deep waters. As such they provide a novel perspective on the source and circulation of deep waters during the Late Cretaceous greenhouse climate including possible changes during ocean anoxic event 2 (OAE2). The fish remains analyzed exhibit good to excellent preservation and show coherent stratigraphic trends across lithologies ranging from carbonaceous black shales to carbonate-poor gray siltstones to red claystones. Unfortunately, fish debris is rare to absent in many samples, and tests are underway to determine whether dispersed Fe-Mn coatings provide a second reliable archive of bottom water values at these sites. At the Cape Verde site (eastern tropical North Atlantic), background εNd values are about -9 through most of the Late Cretaceous, and a positive excursion to -6.5 occurs during OAE2. At the Bermuda Rise site (western subtropical North Atlantic), Late Cretaceous εNd values are -6 to -8 but shift to -10 by the early Paleocene. The OAE2 interval, if present, has not yet been identified at Bermuda Rise. Values of -7 to -9 have been reported for fish debris and other phosphates from shelfal deposits of NW Tethys (Pucéat et al., 2005) and central Tethys (Soudry et al., 2006) suggesting North Atlantic deep waters could have been sourced largely within the North Atlantic-Tethyan region throughout much of the Late Cretaceous. However, the difference in εNd values between sites suggests that there may have been a relatively radiogenic component (e.g., Pacific waters) in deep waters on the west side of the mid-Atlantic Ridge. Further, the OAE2 excursion at the Cape Verde site indicates that at least some deep waters were affected during OAE2. Possible explanations for the εNd excursion include an influx of (or mixing with) radiogenic waters sourced outside the North Atlantic or the input of relatively radiogenic Nd introduced by Caribbean LIP volcanism "upstream" from the Cape Verde Basin. Finally, Paleocene εNd values at Bermuda Rise indicate local deep waters had a strong North Atlantic signature by the early Cenozoic.
PP23C-06
A Holocene Bipolar Seesaw Effect In The South Atlantic?
Almost 10 m of sediments were retrieved from an overgrown lake on Nightingale Island, Tristan da Cunha island group, situated in the centre of the South Atlantic. The sequence comprises 1 m of peat overlain by 8 m of lacustrine sediments with peat on top. It shows a high degree of variability in terms of lithology, geochemistry and pollen composition. More than 50 radiocarbon dates, mainly from terrestrial macro-fossils, form the basis of this high resolution study. They show that after 2 kyr of peat formation a hydrologic shift occurred, possibly when today´s westerlies circulation was established at c. 8.7 kyr BP, and an 8.4 kyr long lake phase followed until the lake grew over 0.3 kyr ago. The lake sediments are exceptionally rich in organic matter, interrupted by recurrent phases of increased mineral content. Such phases coincide with elevated C/N ratios and higher percentages of pollen taxa of upland types as well as decreased sulphur content and less local bog/aquatic pollen types. These periods are interpreted as being characterized by higher precipitation, resulting in increased surface run-off to the lake leading to the observed changes in the proxy records. One of the periods coincides with the Northern Hemisphere 8.2 kyr event. Coupled atmosphere-ocean model simulations of this assumed glacially triggered fresh-water pulse in the Labrador Sea have shown an almost immediate response in the Tristan da Cunha region; dwindling thermohaline circulation (THC) in the north increases sea surface temperatures (SST) in the central South Atlantic resulting in increased air humidity and precipitation, possibly exaggerated by the local orographic island effect. The proxy signature of the assumed response of the 8.2 kyr event on Nightingale Island is more or less identical to at least six other phases, which would imply that all the precipitation-rich periods were triggered by increased SSTs. This is, e.g., supported by our low sulphur values (and high C/S ratios) indicating less influx of sea salt sulphates from marine aerosols, which would increase if the elevated precipitation was caused by intensified westerlies. Phases of inferred increased precipitation, but with a different signature, can also be seen in the bottom peat. All these precipitation-rich phases, assumed to have been caused by high SST´s, are fairly well correlated to periods of increased IRD and dwindling THC in the North Atlantic, especially before 4 kyr BP, implying a bipolar seesaw mechanism in action in the Atlantic Ocean during the Holocene.
PP23C-07
Tectonic Gateways, Circumglobal Currents, and the Stepwise Development of the Great Ocean Conveyor: Inferences From Basinal Benthic Foraminiferal Isotope Syntheses
We have compiled a database of published benthic foraminiferal stable isotope (δ13C, δ18O) records from ODP and DSDP cores. Comparison of separate trends for the Pacific, North Atlantic, South Atlantic/Subantarctic, and high-latitude Southern Oceans provides insight into the development of the strong nutrient (δ13C) and density (δ18O) gradients that characterize the late Neogene conveyor circulation. The transition occurred in a series of four steps: 1) middle Eocene development of a Southern Ocean- to-Pacific/Atlantic density gradient; 2) early Oligocene development of a density gradient between sub-Antarctic and high-latitude Southern Ocean deep water; 3) early Miocene increase in the Southern Ocean-to-Pacific nutrient gradient; 4) middle--late Miocene development of a strong North Atlantic-to-Pacific nutrient gradient. The middle-- late Miocene development of North Atlantic-to-Pacific nutrient gradients has previously been linked to the near- modern influence of North Atlantic deep water and the development of modern conveyor circulation. We link the middle Eocene--Oligocene development of density gradients to the formation of the Antarctic circumpolar current (ACC), initially as a shallow current allowing substantial outflow of Antarctic bottom water (AABW) and subsequently as a deep current and barrier to lateral flow of AABW. The timing of the changes in density gradients is consistent with recent evidence indicating a middle Eocene opening of the Drake passage to shallow flow and deepening of the passage in the early Oligocene. We interpret weak nutrient and density gradients in the pre- middle Eocene Cenozoic as reflecting deep ocean circulation dominated by diffusive transport with insignificant advective currents, and speculate that strong gradients in the Late Cretaceous reflect a Tethyan circumglobal current that, analogous to the ACC, stabilized bipolar deepwater source regions with distinct density characteristics.
PP23C-08
Pacific circulation during the middle Miocene climate transition (12-17 Ma)
We present planktonic and benthic foraminiferal Mg/Ca, δ13C and δ18O as well as census counts and carbonate dissolution proxy data in four ODP cores from the northwestern, central and southeastern Pacific to monitor SST/salinity gradients, thermocline evolution and deep-water ventilation during the middle Miocene climate transition (17-12 Ma). We additionally use Nd isotopes as water mass tracers to track the origins of intermediate and deep water masses during intervals of major climate and circulation change. Our results indicate three main Pacific circulation modes exhibiting different imprints of orbital variations: (1) "Climate Optimum" mode prior to 14.7 Ma characterized by poorly ventilated deep water with prominent 100 kyr variability; (2) transitional mode from 14.7 to 13.9 Ma, principally driven by obliquity and marked by transient improvement in deep-water ventilation as well as surface warming in the subtropical West Pacific; (3) "Icehouse" mode after 13.9 Ma, paced by short eccentricity (100 kyr) and characterized by CCD deepening and increased influence of well ventilated southern sourced deep water. The onset of the transitional and "Icehouse" modes coincided with prominent global positive δ13C excursions and increasing stratification in the subtropical West Pacific (higher δ18O gradients between surface and intermediate/deep waters). Therefore, our results suggest that Antarctic glaciation events were synchronous with the development or expansion of a West Pacific Warm Pool and the establishment of a more vigorous deep water circulation.