Stable isotope analysis of foraminifera from sediment core V23-81 (Table 2)

Oxygen and carbon isotope records from benthic and planktonic foraminifera are presented for the past 35,000 years in the northeastern Atlantic. The results support the idea that the last deglaci-ation took place in two major steps (Duplessy et al., 1981 doi:10.1016/0031-0182(81)90096-1; Mix and Ruddiman, 1985 doi:10.1016/0277-3791(85)90015-0; Ruddiman, 1987; Fairbanks, 1989 doi:10.1038/342637a0), and conflict with theories calling for a strong reduction in North Atlantic deep-water formation to explain the abrupt cooling of the Younger Dryas cold period (Broecker et al., 1985 doi:10.1038/315021a0; Rind et al., 1986 doi:10.1007/BF01277044; Broecker et al., 1988 doi:10.1029/PA003i001p00001).

Stable isotope record during the past 1 Myr of ODP Site 162-980 in the subpolar North Atlantic

Benthic foraminiferal carbon isotope records from a suite of drill sites in the North Atlantic are used to trace variations in the relative strengths of Lower North Atlantic Deep Water (LNADW), Upper North Atlantic Deep Water (UNADW), and Southern Ocean Water (SOW) over the past 1 Myr. During glacial intervals, significant increases in intermediate-to-deep delta13C gradients (commonly reaching >1.2?) are consistent with changes in deep water circulation and associated chemical stratification. Bathymetric delta13C gradients covary with benthic foraminiferal delta18O and covary inversely with Vostok CO2, in agreement with chemical stratification as a driver of atmospheric CO2 changes. Three deep circulation indices based on delta13C show a phasing similar to North Atlantic sea surface temperatures, consistent with a Northern Hemisphere control of NADW/SOW variations. However, lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band.

Oligocene to Miocene stable isotope record and planktonic foraminifer biostratigraphy of the Sierra Leone Rise

We have integrated Oligocene to lower upper Miocene planktonic foraminifer biostratigraphy with benthic foraminifer (Cibicidoides spp.) stable isotope records for two sites drilled on opposite sides of the Sierra Leone Rise in the eastern equatorial Atlantic Ocean. Deep Sea Drilling Project Site 366 (2853 m present water depth; 2200-2800 m paleodepth) recovered an Oligocene to upper Miocene record with a minor unconformity in the "middle" Oligocene and a condensed middle Miocene section. Ocean Drilling Program Site 667 (3529 m present depth; 3000-3500 m paleodepth) recovered an apparently continuous "middle" Oligocene to lower middle Miocene record and a similar condensed middle Miocene section. The Oligocene to lower Miocene sections were deposited at similar sedimentation rates (~11-16 m/m.y.). Stable isotope stratigraphy proved to be useful in establishing intra- and interbasinal correlations. In addition to the well-known earliest Oligocene and middle Miocene S180 increases, a distinct d18O increase occurred near the Oligocene/Miocene boundary. Carbon isotope variations provide similar potential for improving correlations; for example, a d13C increase occurred near the Oligocene/Miocene boundary in concert with increased d18O values. There was little d13C difference between the western Atlantic and eastern Atlantic basins during the late Oligocene and most of the middle Miocene; in contrast, eastern basin d13C values were slightly lower than those in the western basins during the earliest Oligocene (about 35-33 Ma) and early Miocene (about 22-18 Ma).

Benthic foraminiferal stable isotope record for the equatorial Pacific

Stable oxygen and carbon isotope (d18O and d13C) values measured in foraminiferal calcite are one of the primary tools used in paleoceanography. Diagenetic recrystallization of foraminiferal calcite can act to reset primary isotopic values, but its effects are typically poorly quantified. Here we test the impact of early stage diagenesis on stable isotope records generated from a suite of drill sites in the equatorial Pacific Ocean recovered during Ocean Drilling Program Leg 199 and Integrated Ocean Drilling Program Expedition 320. Our selected sites form paleowater and burial depth transects, with excellent stratigraphic control allowing us to confidently correlate our records. We observe large intersite differences in the preservation state of benthic foraminiferal calcite, implying very different recrystallization histories, but negligible intersite offsets in benthic d18O and d13C values. We infer that diagenetic alteration of benthic foraminiferal calcite (in sedimentary oozes) must predominantly occur at shallow burial depths (<100 m) where offsets in both the temperature and isotopic composition of waters in which the foraminifera calcified and pore waters in which diagenesis occurs are small. Our results suggest that even extensive recrystallization of benthic foraminiferal calcite results in minimal shifts from primary d18O and d13C values. This finding supports the long-held suspicion that diagenetic alteration of foraminiferal calcite is less problematic in benthic than in planktic foraminifera and that in deep-sea sediments routinely employed for paleoceanographic studies benthic foraminifera are robust recorders of stable isotope values in the fossil record.

Stable isotope ratios of epibenthic foraminifer Cibicidoides wuellerstorfi from sediment profile GeoB9526

Studies of temporal changes of ocean circulation and deep-water ventilation often rely on d13C records of epibenthic foraminifer Cibicidoides wuellerstorfi. However, primary productivity related overprints may distort the signal and simulate a chemical age of ambient water mass that is too old and simulates poorly ventilated ambient bottom waters. To further constrain the use of C. wuellerstorfi d13C records from high-productivity areas, we analyzed a 14CAMS-dated gravity core from the upwelling regime off northwest Africa at 12°N. We compare this new record with 37 radiocarbon dated d13C records from the eastern Atlantic Ocean between 45°N and 25°S that are bathed by the same water mass. Only during Heinrich events 1 and 2, when the investigated core site off northwest Africa experienced year-round, sustained deposition of organic matter, the d13C values at this site faithfully record deep-water ventilation states. During times of predominantly seasonal deposition of fresh phytodetritus, however, d13C values were significantly lower than at the reference sites. This underscores that reconstruction of paleocirculation and deep ocean ventilation using C. wuellerstorfi d13C from regions that experienced seasonal phytodetritus deposition needs to be validated by additional proxies that are not affected by local productivity.

Pliocene-Pleistocene stable isotope record of ODP Site 107-653

Planktonic foraminiferal oxygen and carbon isotope analyses from Tyrrhenian Sea Ocean Drilling Program (ODP) Site 653 provide a continuous record of the Pliocene-Pleistocene paleoceanographic history of the Mediterranean. Long-term trends in oxygen isotopes primarily reflect changes in global climatic conditions, with a more local or regional signal superimposed on this record. For example, significant enrichments in 18O due to decreases in surface water temperature and/or increases in continental ice volume occurred at 3.1, 2.7, 2.1, 1.6, and 0.4 Ma. In contrast to most open-ocean results, the early Pliocene 6lsO record of Site 653 exhibits high-amplitude fluctuations indicative of very unstable climatic conditions in this region. Another unique aspect of this Mediterranean d18Orecord is the pronounced cooling at the Pliocene/Pleistocene boundary. The carbon isotope record for Site 653 also exhibits high-amplitude variability throughout the Pliocene-Pleistocene. This variability most probably reflects changes in the carbon isotopic composition of the source of Mediterranean surface waters.

Results of an in situ stable isotope labeling study in intertidal sediment

We report the results of an in situ tracer experiment in an intertidal sediment, where bacterial carbon was tagged with stable carbon-isotope label, after the injection of 13C-glucose. The appearance of label in bacteria (based on label incorporation in bacteria-specific, phospholipid-derived fatty acids) and subsequent transfer to meiobenthos (group level) and macrobenthos (species level) was followed for 36 days. The label dynamics of benthic taxa were either fitted with a simple-isotope model or evaluated against enrichment in bacteria, to derive the importance of bacterially derived carbon for the meiobenthos and macrobenthos. Although selective uptake of bacteria was evident, as 2.4 times more bacterial carbon was grazed as expected from indiscriminate feeding, bacterial carbon accounted on average for only 0.08 and 0.11 of the carbon requirements of meiobenthic and macrobenthic taxa, respectively. Additionally, the contribution of bacterial carbon to total carbon requirements did not depend on the living/feeding depth in the sediment or organism size (evaluated over a size range of four orders of magnitude). The observed overall low contribution of bacterial carbon implies that most intertidal benthic fauna depend primarily on other carbon resources that may assert a stronger control on the structure of intertidal-sediment communities.

Geochemistry and stable isotope record of black shales and inoceramids from the Demerara Rise

The bulk rock geochemistry and inoceramid isotopic composition from Cenomanian to Santonian, finely laminated, organic-rich black shales, recovered during Ocean Drilling Program Leg 207 on Demerara Rise (western tropical North Atlantic), suggest persistent anoxic (free H2S) conditions within the sediments and short-term variations within a narrow range of anoxic to episodically dysoxic bottom waters over a ~15 Ma time interval. In addition to being organic-rich, the 50-90 m thick sections examined exhibit substantial bulk rock enrichments of Si, P, Ba, Cu, Mo, Ni, and Zn relative to World Average Shale. These observations point to high organic burial fluxes, likely driven by high primary production rates, which led to the establishment of intensely sulfidic pore waters and possibly bottom waters, as well as to the enrichments of Cr, Mo, U, and V in the sediments. At the same time, the irregular presence of benthic inoceramids and foraminifera in this facies demonstrates that the benthic environment could not have been continuously anoxic. The d13C and d15N values of the inoceramid shell organics provide no evidence of chemosymbiosis and are consistent with pelagic rain as being a significant food source. Demerara Rise inoceramids also exhibit well-defined, regularly spaced growth lines that are tracked by d13C and d18O variations in shell carbonate that cannot be simply explained by diagenesis. Instead, productivity variations in surface waters may have paced the growth of the shells during brief oxygenation events suitable for benthic inoceramid settlement. These inferences imply tight benthopelagic coupling and more dynamic benthic conditions than generally portrayed during black shale deposition. By invoking different temporal scales for geochemical and paleontological data, this study resolves recent contradictory conclusions (e.g., sulfidic sedimentary conditions versus dysoxic to suboxic benthic waters) drawn from studies of either sediment geochemistry or fossil distributions alone on Demerara Rise. This variability may be relevant for discussions of black shales in general.

Miocene stable isotope record of IODP Site 320-U1335 and revised stable isotope record of IODP Site 321-U1337

The Miocene Climatic Optimum (MCO; ~16.9 to 14.7 Ma) provides an outstanding opportunity to investigate climate-carbon cycle dynamics during a geologically recent interval of global warmth. We present benthic stable oxygen (d18O) and carbon (d13C) isotope records (5-12 kyr time resolution) spanning the late early to middle Miocene interval (18 to 13 Ma) at Integrated Ocean Drilling Program (IODP) Site U1335 (eastern equatorial Pacific Ocean). The U1335 stable isotope series track the onset and development of the MCO as well as the transitional climatic phase culminating with global cooling and expansion of the East Antarctic ice-sheet at ~13.8 Ma. We integrate these new data with published stable isotope, geomagnetic polarity and X-ray fluorescence (XRF) scanner-derived carbonate records from IODP Sites U1335, U1336, U1337 and U1338 on a consistent, astronomically-tuned timescale. Benthic isotope and XRF scanner-derived CaCO3 records depict prominent 100 kyr variability with 400 kyr cyclicity additionally imprinted on d13C and CaCO3 records, pointing to a tight coupling between the marine carbon cycle and climate variations. Our inter-site comparison further indicates that the lysocline behaved in highly dynamic manner throughout the MCO, with >75% carbonate loss occurring at paleo-depths ranging from ~3.4 to ~4 km in the eastern equatorial Pacific Ocean. Carbonate dissolution maxima coincide with warm phases (d18O minima) and d13C decreases, implying that climate-carbon cycle feedbacks fundamentally differed from the late Pleistocene glacial-interglacial pattern, where dissolution maxima correspond to d13C maxima and d18O minima. Carbonate dissolution cycles during the MCO were, thus, more similar to Paleogene hyperthermal patterns.