Stable carbon isotope record of sediment core Ancora

The standard paradigm that the Paleocene/Eocene thermal maximum (PETM) represents a threshold event intrinsic to Earth's climate and connected in some way with long-term warming has influenced interpretations of the geochemical, climate, and biological perturbations that occurred at this event. As recent high-resolution data have demonstrated that the onset of the event was geologically instantaneous, attempts to account for the event solely through endogenous mechanisms have become increasingly strained. The rapid onset of the event indicates that it was triggered by a catastrophic event which we suggest was most likely a bolide impact. We discuss features of the PETM that require explanation and argue that mechanisms that have previously been proposed either cannot explain all of these features or would require some sort of high-energy trigger. A bolide impact could provide such a trigger and, in the event of a comet impact, could contribute directly to the shape of the carbon isotope curve. We introduce a carbon cycle model that would explain the PETM by global warming following a bolide impact, leading to the oxidation of terrestrial organic carbon stores built up during the late Paleocene. Our intention is to encourage other researchers to seriously consider an impact trigger for the PETM, especially in the absence of plausible alternative mechanisms.

Stable carbon isotope record of sediment core Bass_River_Site

A thick, apparently continuous section recording events of the latest Paleocene thermal maximum in a neritic setting was drilled at Bass River State Forest, New Jersey as part of ODP Leg 174AX [Miller, Sugarman, Browning et al., 1998]. Integrated nannofossil and magneto-stratigraphy provides a firm chronology supplemented by planktonic foraminiferal biostratigraphy. This chronologic study indicates that this neritic section rivals the best deep-sea sections in providing a complete record of late Paleocene climatic events. Carbon and oxygen isotopes measured on benthic foraminifera show a major (4.0% in carbon, 2.3% in oxygen) negative shift correlative with the global latest Paleocene carbon isotope excursion (CIE). A sharp increase in kaolinite content coincides with the isotope shift in the Bass River section, analogous to increases found in several other records. Carbon and oxygen isotopes remain low and kaolinite content remains high for the remainder of the depositional sequence above the CIE (32.5 ft, 9.9 m), which we estimate to represent 300-500 k.y. We interpret these data as indicative of an abrupt shift to a warmer and wetter climate along the North American mid-Atlantic coast, in concert with global events associated with the CIE.

Stable isotope ratios, carbon content and primary production of three sediment cores from the subantarctic eastern Atlantic

We present three new benthic foraminiferal delta13C, delta18O, and total organic carbon time series from the eastern Atlantic sector of the Southern Ocean between 41°S and 47°S. The measured glacial delta13C values belong to the lowest hitherto reported. We demonstrate a coincidence between depleted late Holocene (LH) delta13C values and positions of sites relative to ocean surface productivity. A correction of +0.3 to +0.4 [per mil VPDB] for a productivity-induced depletion of Last Glacial Maximum (LGM) benthic delta13C values of these cores is suggested. The new data are compiled with published data from 13 sediment cores from the eastern Atlantic Ocean between 19°S and 47°S, and the regional deep and bottom water circulation is reconstructed for LH (4-0 ka) and LGM (22-16 ka) times. This extends earlier eastern Atlantic-wide synoptic reconstructions which suffered from the lack of data south of 20°S. A conceptual model of LGM deep-water circulation is discussed that, after correction of southernmost cores below the Antarctic Circumpolar Current (ACC) for a productivity-induced artifact, suggests a reduced formation of both North Atlantic Deep Water in the northern Atlantic and bottom water in the southwestern Weddell Sea. This reduction was compensated for by the formation of deep water in the zone of extended winter sea-ice coverage at the northern rim of the Weddell Sea, where air-sea gas exchange was reduced. This shift from LGM deep-water formation in the region south of the ACC to Holocene bottom water formation in the southwestern Weddell Sea, can explain lower preformed d13CDIC values of glacial circumantarctic deep water of approximately 0.3 per mil to 0.4 per mil. Our reconstruction brings Atlantic and Southern Ocean d13C and Cd/Ca data into better agreement, but is in conflict, however, with a scenario of an essentially unchanged thermohaline deep circulation on a global scale. Benthic delta18O-derived LGM bottom water temperatures, by 1.9°C and 0.3°C lower than during the LH at deepest southern and shallowest northern sites, respectively, agree with the here proposed reconstruction of deep-water circulation in the eastern South Atlantic Ocean.

Stable carbon and oxygen isotope ratios of benthic foraminifera

Carbon isotopic measurements on the benthic foraminiferal genus Cibicidoides document that mean deep ocean delta13C values were 0.46 per mil lower during the last glacial maximum than during the Late Holocene. The geographic distribution of delta13C was altered by changes in the production rate of nutrient-depleted deep water in the North Atlantic. During the Late Holocene, North Atlantic Deep Water, with high delta13C values and low nutrient values, can be found throughout the Atlantic Ocean, and its effects can be traced into the southern ocean where it mixes with recirculated Pacific deep water. During the glaciation, decreased production of North Atlantic Deep Water allowed southern ocean deep water to penetrate farther into the North Atlantic and across low-latitude fracture zones into the eastern Atlantic. Mean southern ocean delta13C values during the glaciation are lower than both North Atlantic and Pacific delta13C values, suggesting that production of nutrient-depleted water occurred in both oceans during the glaciation. Enriched 13C values in shallow cores within the Atlantic Ocean indicate the existence of a nutrient-depleted water mass above 2000 m in this ocean.

Seismic reflection data, calcium/iron ratios, stable istopes, and magnetic susceptibility of sediment cores from the Pen Duick Escarpment, Gulf of Cadiz

Here we present a case study of three cold-water coral mounds in a juvenile growth stage on top of the Pen Duick Escarpment in the Gulf of Cadiz; Alpha, Beta and Gamma mounds. Although cold-water corals are a common feature on the adjacent cliffs, mud volcanoes and open slope, no actual living cold-water coral has been observed. This multidisciplinary and integrated study comprises geophysical, sedimentological and (bio)geochemical data and aims to present a holistic view on the interaction of both environmental and geological drivers in cold-water coral mound development in the Gulf of Cadiz. Coring data evidences (past or present) methane seepage near the Pen Duick Escarpment. Several sources and pathways are proposed, among which a stratigraphic migration through uplifted Miocene series underneath the escarpment. The dominant morphology of the escarpment has influenced the local hydrodynamics within the course of the Pliocene, as documented by the emplacement of a sediment drift. Predominantly during post-Middle Pleistocene glacial episodes, favourable conditions were present for mound growth. An additional advantage for mound formation near the top of Pen Duick Escarpment is presented by seepage-related carbonate crusts which might have offered a suitable substrate for coral settling. The spatially and temporally variable character and burial stage of the observed open reef frameworks, formed by cold-water coral rubble, provides a possible model for the transition from cold-water coral reef patches towards juvenile mound. These rubble "graveyards" not only act as sediment trap but also as micro-habitat for a wide range of organisms. The presence of a fluctuating Sulphate-Methane Transition Zone has an important effect on early diagenetic processes, affecting both geochemical and physical characteristics, transforming the buried reef into a solid mound. Nevertheless, the responsible seepage fluxes seem to be locally variable. As such, the origin and evolution of the cold-water coral mounds on top of the Pen Duick Escarpment is, probably more than any other NE Atlantic cold-water coral mound province, located on the crossroads of environmental (hydrodynamic) and geological (seepage) pathways.

(Table 1) Oxygen and carbon isotopes for foraminiferal taxa at DSDP Hole 76-540

We analyzed the oxygen and carbon isotopic composition of planktonic and benthic foraminifers picked from 13 late Eocene to late Oligocene samples from DSDP Site 540 (23°49.73'N, 84°22.25'W, 2926 m water depth) from the Gulf of Mexico. An enrichment occurs in 18O of about 0.5 to 0.8 per mil in both benthic foraminifers and surface-dwelling planktonic foraminifers between the latest Eocene and early Oligocene. This early Oligocene maximum is followed by lower 18O values. A 1.2 per mil d13C decrease in both benthic and planktonic foraminiferal data occurs from the late Eocene to the late Oligocene. There is a correspondence of the 13C signal to deep-sea records; however, the amplitude of this change is greater than previously seen in deep-sea cores, possibly as a result of proximity to terrestrial sources of carbon. The covarying isotopic changes in both benthic and planktonic foraminifers suggest global causes, such as ice volume increases and increased terrestrial carbon input to the ocean. However, during the latter part of the record (early-late Oligocene), the increases in the benthic 18O without accompanying increases observed with planktonic foraminifers suggest that changes in only one part of the system occurred; one potential explanation being a decrease in bottom-water temperatures without concomitant changes in the surface waters. The 18O differences between species of planktonic foraminifers and the difference between planktonic and benthic 18O data indicate that diagenesis problems are minimal. These preliminary results are encouraging given that these cores are partially lithified.

Stable isotope record, calcium carbonate conentrations, and sea surface temperture reconstructions of sediment cores from the North Atlantic

We analyze five high-resolution time series spanning the last 1.65 m.y.: benthic foraminiferal delta18O and delta13O, percent CaCO3, and estimated sea surface temperature (SST) at North Atlantic Deep Sea Drilling Project site 607 and percent CaCO3 at site 609. Each record is a multicore composite verified for continuity by splicing among multiple holes. These climatic indices portray changes in northern hemisphere ice sheet size and in North Atlantic surface and deep circulation. By tuning obliquity and precession components in the delta18O record to orbital variations, we have devised a time scale (TP607) for the entire Pleistocene that agrees in age with all K/Ar-dated magnetic reversals to within 1.5%. The Brunhes time scale is taken from Imbrie et al. [1984], except for differences near the stage 17/16 transition (0.70 to 0.64 Ma). All indicators show a similar evolution from the Matuyama to the Brunhes chrons: orbital eccentricity and precession responses increased in amplitude; those at orbital obliquity decreased. The change in dominance from obliquity to eccentricity occurred over several hundred thousand years, with fastest changes around 0.7 to 0.6 Ma. The coherent, in-phase responses of delta18O, delta13O, CaCO3 and SST at these rhythms indicate that northern hemisphere ice volume changes have controlled most of the North Atlantic surface-ocean and deep-ocean responses for the last 1.6 m.y. The delta13O, percent CaCO3, and SST records at site 607 also show prominent changes at low frequencies, including a prominent long-wavelength oscillation toward glacial conditions that is centered between 0.9 and 0.6 Ma. These changes appear to be associated neither with orbital forcing nor with changes in ice volume.

Cd and d13C reconstruction for the LGM and Holocene in Brazil Margin sediment cores

Paleoceanographic studies using benthic foraminiferal Cd as a nutrient tracer have provided a robust means of reconstructing glacial Atlantic Ocean water mass geometry, but a paucity of data from the South Atlantic above 1200 m has limited investigation of Antarctic Intermediate Water (AAIW) configuration and formation. A new Cd depth profile from Brazil margin sediments suggests that AAIW penetrated northward at 1100 m to at least 27°S in the glacial Atlantic. It exhibited substantially reduced d13Cas values, confirming preliminary evidence that this AAIW was unique to the glacial Atlantic and that it formed differently than today, with less atmospheric contact.