CaCO3, foraminifer fragmentation, benthic carbon isotopes, coarse lithic counts and radiogenic isotopes (Sr, Nd, Pb) of the Pliocene and earliest Pleistocene terrigenous component (3.3-2.4 Ma) of IODP Site 306-U1313

We present Plio-Pleistocene records of sediment color, %CaCO3, foraminifer fragmentation, benthic carbon isotopes (d13C) and radiogenic isotopes (Sr, Nd, Pb) of the terrigenous component from IODP Site U1313, a reoccupation of benchmark subtropical North Atlantic Ocean DSDP Site 607. We show that (inter)glacial cycles in sediment color and %CaCO3 pre-date major northern hemisphere glaciation and are unambiguously and consistently correlated to benthic oxygen isotopes back to 3.3 million years ago (Ma) and intermittently so probably back to the Miocene/Pliocene boundary. We show these lithological cycles to be driven by enhanced glacial fluxes of terrigenous material (eolian dust), not carbonate dissolution (the classic interpretation). Our radiogenic isotope data indicate a North American source for this dust (~3.3-2.4 Ma) in keeping with the interpreted source of terrestrial plant wax-derived biomarkers deposited at Site U1313. Yet our data indicate a mid latitude provenance regardless of (inter)glacial state, a finding that is inconsistent with the biomarker-inferred importance of glaciogenic mechanisms of dust production and transport. Moreover, we find that the relation between the biomarker and lithogenic components of dust accumulation is distinctly non-linear. Both records show a jump in glacial rates of accumulation from Marine Isotope Stage, MIS, G6 (2.72 Ma) onwards but the amplitude of this signal is about 3-8 times greater for biomarkers than for dust and particularly extreme during MIS 100 (2.52 Ma). We conclude that North America shifted abruptly to a distinctly more arid glacial regime from MIS G6, but major shifts in glacial North American vegetation biomes and regional wind fields (exacerbated by the growth of a large Laurentide Ice Sheet during MIS 100) likely explain amplification of this signal in the biomarker records. Our findings are consistent with wetter-than-modern reconstructions of North American continental climate under the warm high CO2 conditions of the Early Pliocene but contrast with most model predictions for the response of the hydrological cycle to anthropogenic warming over the coming 50 years (poleward expansion of the subtropical dry zones).

Oxygen- and carbon-isotopes at DSDP Leg 73 Holes

We established a composite oxygen- and carbon-isotope stratigraphy for the Pliocene in the central South Atlantic. Monospecific samples of benthic and planktonic foraminifers from pelagic sediments from DSDP Sites 519, 521, 522, and 523 were analyzed isotopically. The resulting benthic oxygen-isotope stratigraphy allowed three paleoclimatic periods in the Pliocene to be distinguished. During the early Pliocene (5.2-3.3 Ma), low-amplitude climatic changes prevailed in a world that was less glaciated than during the Pleistocene. A net increase in global ice volume is documented in a 0.5 permil positive shift in the average 18O composition of the benthic foraminifers at 3.2 Ma. The middle Pliocene (3.3-2.5 Ma) is not only characterized by a more widespread glaciation of the Southern and Northern hemispheres but also by more drastic isotopic differences between glacial and interglacial times. A minor shift in the average 18O composition of the benthic foraminifers marks the beginning of the late Pliocene-early Pleistocene climatic period (2.5-1.1 Ma). Alternating cold and warm climate is documented in both the oxygen-isotope record and in the pelagic sediments. During cold periods, sediments with a lower CaCO3 content indicate more corrosive bottom-water conditions. More negative 13C signals in the benthic foraminifers from these sediments suggest that the Antarctic Bottom Water current was intensified in glacial times. The oxygen-isotope composition of the measured planktonic foraminifers suggests that the surface water in this part of the South Atlantic remained relatively warm during the growth of the Pliocene glaciers.

Stable nitrogen and carbon isotopes of plankton samples and selected copoepods around the Iberan Peninsula 2001-2004

The structure and variability of pelagic food webs along the north and northwestern shelf of the Iberian Peninsula were analysed using natural abundance of nitrogen stable isotopes of plankton and pelagic consumers. Plankton composition was mainly studied in size-fractionated samples, but also the isotopic signatures of three copepod species, as representative of primary consumers, were considered. Several fish species were included as planktivorous consumers, with special attention to sardine (Sardina pilchardus). Finally, top pelagic consumers were represented by the common dolphin (Delphinus delphis). The relationship between trophic position and body size implies large variability in the ratio of predator to prey sizes, likely because widespread omnivory and plankton consumption by relatively large predators. Planktivorous species share a common trophic position, suggesting potential competition for food, and low nitrogen isotope enrichment between prey and consumers suggest nutrient limitation and recycling at the base of the food web. Both experimental and field evidences indicate that the muscle of sardine integrates fish diet over seasonal periods and reflects the composition of plankton from large shelf areas. The low mobility of sardines during periods of low population size is consistent with differential isotopic signatures found in shelf zones characterised by upwelling nutrient inputs.

Oxygen and carbon isotopes of planktic foraminifera and ageprofile datum description at DSDP Hole 69-504

A detailed oxygen isotope record (resolution: about 2500 years) has been obtained for the Pleistocene sediments at Hole 504. Preliminary measurements made deeper in the section suggest that at least the upper Pliocene section is also amenable to detailed stable isotope work. The record for the middle Pleistocene resembles that obtained previously from piston cores in the western equatorial Pacific, although the superior resolution of this high-accumulation-rate site reveals a greater amplitude of isotope variation than previously observed. The record for the lower Pleistocene reveals variation that is both greater in amplitude and higher in frequency than apparent from previously analyzed piston cores. The site provides the best material recovered to date for the study of the evolution of climatic variability during the past few million years.

Oxygen- and carbon-isotopes of Globigerina bulloides and Neogloboquadrina pachyderma at DSDP Holes 80-548 and 80-549A

Oxygen- and carbon-isotope analyses have been performed on the Quaternary planktonic foraminifers of Sites 548 and 549 (DSDP Leg 80) to investigate major water mass changes that occurred in the northeastern Atlantic at different glacial-interglacial cycles and to compare them with the well-defined picture of 18,000 yr. ago. Oxygen-isotope stratigraphy also provides a chronological framework for the more important data on the fauna and flora. Although bioturbation and sedimentary gaps obliterate the climatic and stratigraphic record, general trends in the oceanographic history can be deduced from the isotopic data. Isotopic stratigraphy has tentatively been delineated down to isotopic Stage 16 at Site 548 and in Hole 549A. This stratigraphy fits well with that deduced from benthic foraminiferal d18O changes and with bioclimatic zonations based on foraminiferal associations at Site 549. Variations in the geographic extension and in the flux of the Gulf Stream subtropical waters are inferred from both d18O and d13C changes. Maximal fluxes occurred during the late Pliocene. Northward extension of subtropical waters increased through the various interglacial phases of the early Pleistocene and decreased through the late Pleistocene interglacial phases. Conversely, glacial maxima were more intense after Stage 16. Isotopic Stages 12 and 16 mark times of important change in water mass circulation. Oxygen- and carbon-isotope analyses have been performed on the Quaternary planktonic foraminifers of Sites 548 and 549 (DSDP Leg 80) to investigate major water mass changes that occurred in the northeastern Atlantic at different glacial-interglacial cycles and to compare them with the well-defined picture of 18,000 yr. ago. Oxygen-isotope stratigraphy also provides a chronological framework for the more important data on the fauna and flora. Although bioturbation and sedimentary gaps obliterate the climatic and stratigraphic record, general trends in the oceanographic history can be deduced from the isotopic data. Isotopic stratigraphy has tentatively been delineated down to isotopic Stage 16 at Site 548 and in Hole 549A. This stratigraphy fits well with that deduced from benthic foraminiferal d18O changes and with bioclimatic zonations based on foraminiferal associations at Site 549. Variations in the geographic extension and in the flux of the Gulf Stream subtropical waters are inferred from both d18O and d13C changes. Maximal fluxes occurred during the late Pliocene. Northward extension of subtropical waters increased through the various interglacial phases of the early Pleistocene and decreased through the late Pleistocene interglacial phases. Conversely, glacial maxima were more intense after Stage 16. Isotopic Stages 12 and 16 mark times of important change in water mass circulation.