Modeling the sensitivity of precipitation oxygen isotopes to the Last Glacial Maximum boundary conditions: A study using Community Atmosphere Model (CAM3.0)

To understand the validity of d18O proxy records as indicators of past temperature change, a series of experiments was conducted using an atmospheric general circulation model fitted with water isotope tracers (Community Atmosphere Model version 3.0, IsoCAM). A pre-industrial simulation was performed as the control experiment, as well as a simulation with all the boundary conditions set to Last Glacial Maximum (LGM) values. Results from the pre-industrial and LGM simulations were compared to experiments in which the influence of individual boundary conditions (greenhouse gases, ice sheet albedo and topography, sea surface temperature (SST), and orbital parameters) were changed each at a time to assess their individual impact. The experiments were designed in order to analyze the spatial variations of the oxygen isotopic composition of precipitation (d18Oprecip) in response to individual climate factors. The change in topography (due to the change in land ice cover) played a significant role in reducing the surface temperature and d18Oprecip over North America. Exposed shelf areas and the ice sheet albedo reduced the Northern Hemisphere surface temperature and d18Oprecip further. A global mean cooling of 4.1 °C was simulated with combined LGM boundary conditions compared to the control simulation, which was in agreement with previous experiments using the fully coupled Community Climate System Model (CCSM3). Large reductions in d18Oprecip over the LGM ice sheets were strongly linked to the temperature decrease over them. The SST and ice sheet topography changes were responsible for most of the changes in the climate and hence the d18Oprecip distribution among the simulations.

(Table 1) Geochemistry of ODP Hole 130-807C carbonates across the K/T boundary

Paleontological, stable isotopic, trace elemental abundance, and magnetostratigraphic studies have been performed on limestones spanning the Cretaceous/Tertiary boundary transition at Ocean Drilling Program (ODP) Hole 807C. Paleontological evidence exists for considerable resedimentation, which we attribute to the fact that Hole 807C is located in a basement graben. Age estimates based on planktonic foraminiferal biostratigraphy, as well as magnetostratigraphy, indicate that sedimentation rates could have been on the order of 12-14 m/m.y. This is significantly higher than those documented in other important Deep Sea Drilling Project (DSDP) and ODP Cretaceous/Tertiary boundary sections using the same age control points (e.g., DSDP Hole 577 and ODP Hole 690B), although not as high as those documented from DSDP Hole 524. The expanded nature of this succession has resulted in the Cretaceous/Tertiary boundary d13C decrease occurring over approximately a 9-m interval. Ir analysis of these sediments do not show a single large anomaly, as has been found in other Cretaceous/Tertiary boundary sections, but trivial background levels instead. Ce data support the hypothesis that this section has been expanded by secondary sedimentological processes.

Three time-slice simulations of the Eemian (eem), the mid-Holocene (hol), and a pre-industrial (pri) control run with the coupled atmosphere-ocean-model ECHAM5/MPI-OM

Orbital forcing does not only exert direct insolation effects, but also alters climate indirectly through feedback mechanisms that modify atmosphere and ocean dynamics and meridional heat and moisture transfers. We investigate the regional effects of these changes by detailed analysis of atmosphere and ocean circulation and heat transports in a coupled atmosphere-ocean-sea ice-biosphere general circulation model (ECHAM5/JSBACH/MPI-OM). We perform long term quasi equilibrium simulations under pre-industrial, mid-Holocene (6000 years before present - yBP), and Eemian (125 000 yBP) orbital boundary conditions. Compared to pre-industrial climate, Eemian and Holocene temperatures show generally warmer conditions at higher and cooler conditions at lower latitudes. Changes in sea-ice cover, ocean heat transports, and atmospheric circulation patterns lead to pronounced regional heterogeneity. Over Europe, the warming is most pronounced over the north-eastern part in accordance with recent reconstructions for the Holocene. We attribute this warming to enhanced ocean circulation in the Nordic Seas and enhanced ocean-atmosphere heat flux over the Barents Shelf in conduction with retreat of sea ice and intensified winter storm tracks over northern Europe.

Os isotope ratios of sediments from the Eocene-Oligocene boundary

Osmium (Os) isotope analyses of bulk sediments from the South Atlantic, Equatorial Pacific, and the Italian Apennines yield a well-dated and coherent pattern of 187Os/188Os variation from the late Eocene to the early Oligocene. The resulting composite record demonstrates the global character of two prominent features of the low-resolution LL44-GPC3 Os isotope record (Pegram and Turekian, 1999, doi:10.1016/S0016-7037(99)00308-7). These are: (1) a pronounced minimum in 187Os/188Os (0.22-0.27) in the late Eocene, between 34 and 34.5 Ma, and (2) a subsequent rapid increase in 187Os/188Os, to approximately 0.6 by 32 Ma. An ultramafic weathering event and an increased influx of extraterrestrial particles to the Earth are discussed as alternative explanations for the late Eocene 187Os/188Os minimum. Comparison of the 187Os/188Os to benthic foraminiferal oxygen isotope records demonstrates that the nearly three-fold increase in 187Os/188Os from the late Eocene minimum coincides with the growth and decay of the first large ice sheet of the Oligocene (Oi1 (Miller et al., 1991, doi:10.1029/90JB02015)). The fine structure of the Os isotope record indicates that enhanced release of radiogenic Os, unrelated to the recovery from late Eocene minimum, lagged the initiation of the Oi1 event by roughly 0.5 Myr. This record, in conjunction with weathering studies in modern glacial soils (Blum, in: W.F. Ruddiman (Ed.), Tectonic Uplift and Climate Change, Plenum Press, New York, 1997, pp. 259-288; Peucker-Ehrenbrink and Blum, 1998, doi:10.1016/S0016-7037(98)00227-0), suggests that exposure of freshly eroded material during deglaciation following Oi1 enhanced chemical weathering rates, and may have contributed to ice sheet stabilization by drawing down atmospheric carbon dioxide. The improved temporal resolution and age control of the refined Eocene-Oligocene Os isotope record also makes it possible to illustrate the late Eocene Os isotope excursion as a tool for global correlation of marine sediments.

Palynological analyses of the sediments around the Miocene/Pliocene boundary in ODP Hole 151-911A

The late Neogene evolution of the Arctic to Subarctic region is poorly understood due to few available records and poor age control. At the margin of the Arctic Ocean, Yermak Plateau Ocean Drilling Program (ODP) Hole 911A is strategically located for establishing a stratigraphic framework for the Arctic. Here we present dinoflagellate cyst and acritarch data from 24 stratigraphic levels in the lower part (474.26-505.64 metres below the seafloor (mbsf)) of ODP Hole 911A. The marine palynomorphs indicate a latest Miocene to earliest Pliocene age (between 5.8 and 5.0 Ma) for the base of the hole based on the co-occurrence of the dinoflagellate cyst Barssidinium evangelineae and acritarch Lavradosphaera crista. Our age estimate for the sediments can possibly be further refined to 5.0-5.33 Ma based on the presence of Achomosphaera andalousiensis suttonensis, which apparently has a range restricted to the Pliocene. An age close to the Miocene/Pliocene boundary agrees with the planktonic foraminifer data. Together with recently available magnetostratigraphic data, the base of the hole is likely to be placed at ~5.2 Ma. This new chronostratigraphy is a first step towards a better understanding of the late Neogene palaeoenvironment for the Yermak Plateau and also for the wider Arctic to Subarctic region. The terrestrial and fresh water palynomorphs were most likely redistributed and/or displaced from the shelf towards deeper parts of the basin during contourite deposition under the influence of the West Spitsbergen Current. The in situ marine dinoflagellate cyst assemblage contains a mixture of cool water and thermophilic taxa, indicating sea-ice free, cool-temperate, warmer than present conditions at the Yermak Plateau. Rivers were likely the source for the freshwater influence.

(Table 2) Depth-age control points of ODP Site 177-1090

At Ocean Drilling Program (ODP) Site 1090 on the Agulhas Ridge (subantarctic South Atlantic) benthic foraminiferal stable isotope records span the late Oligocene through the early Miocene (25~16 Ma) at a temporal resolution of ?10 kyr. In the same time interval a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity timescale (GPTS), thereby providing secure correlation of the isotope record to the GPTS. On the basis of the isotope-magnetostratigraphic correlation we provide refined age calibration of established oxygen isotope events Mi1 through Mi2 as well as several other distinctive isotope events. Our data suggest that the d18O maximum commonly associated with the Oligocene/Miocene (O/M) boundary falls within C6Cn.2r (23.86 Ma). The d13C maximum coincides, within the temporal resolution of our record, with C6Cn.2n/r boundary and hence to the O/M boundary. Comparison of the stable isotope record from ODP Site 1090 to the orbitally tuned stable isotope record from ODP Site 929 across the O/M boundary shows that variability in the two records is very similar and can be correlated at and below the O/M boundary. Site 1090 stable isotope records also provide the first deep Southern Ocean end-member for reconstructions of circulation patterns and late Oligocene to early Miocene climate change. Comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow. Oxygen isotope gradients between sites suggest that the deep Southern Ocean was cold in comparison to the North Atlantic, Indian, and the Pacific Oceans. Dominance of cold Southern Component Deep Water at Site 1090, at least until 17 Ma, suggests that relatively cold circumpolar climatic conditions prevailed during the late Oligocene and early Miocene. We believe that a relatively cold Southern Ocean reflects unrestricted circumpolar flow through the Drake Passage in agreement with bathymetric reconstructions.

Magnetic suceptibility, carbon and oxygen stable isotope ratios and reflectance from the Bottaccione Gorge and Furlo section, Italy

The oceans at the time of the Cenomanian-Turonian transition were abruptly perturbed by a period of bottom-water anoxia. This led to the brief but widespread deposition of black organic-rich shales, such as the Livello Bonarelli in the Umbria-Marche Basin (Italy). Despite intensive studies, the origin and exact timing of this event are still debated. In this study, we assess leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world, by providing a 6-Myr-long astronomically-tuned timescale across the Cenomanian-Turonian boundary. We procure insights in the relationship between orbital forcing and the Late Cretaceous carbon cycle by deciphering the imprint of astronomical cycles on lithologic, geophysical, and stable isotope records, obtained from the Bottaccione, Contessa and Furlo sections in the Umbria-Marche Basin. The deposition of black shales and cherts, as well as the onset of oceanic anoxia, is related to maxima in the 405-kyr cycle of eccentricity-modulated precession. Correlation to radioisotopic ages from the Western Interior (USA) provides unprecedented age control for the studied Italian successions. The most likely tuned age for the Livello Bonarelli base is 94.17 ± 0.15 Ma (tuning #1); however, a 405-kyr older age cannot be excluded (tuning #2) due to uncertainties in stratigraphic correlation, radioisotopic dating, and orbital configuration. Our cyclostratigraphic framework suggests that the exact timing of major carbon cycle perturbations during the Cretaceous may be linked to increased variability in seasonality (i.e. a 405-kyr eccentricity maximum) after the prolonged avoidance of seasonal extremes (i.e. a 2.4-Myr eccentricity minimum). Volcanism is probably the ultimate driver of oceanic anoxia, but orbital periodicities determine the exact timing of carbon cycle perturbations in the Late Cretaceous. This unites two leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world.