Oxygen isotopes, IRD and SST reconstruction for the interval MIS31-19 of IODP Site 306-U1314

Early and Mid-Pleistocene climate, ocean hydrography and ice sheet dynamics have been reconstructed using a high-resolution data set (planktonic and benthic d18O time series, faunal-based sea surface temperature (SST) reconstructions and ice-rafted debris (IRD)) record from a high-deposition-rate sedimentary succession recovered at the Gardar Drift formation in the subpolar North Atlantic (Integrated Ocean Drilling Program Leg 306, Site U1314). Our sedimentary record spans from late in Marine Isotope Stage (MIS) 31 to MIS 19 (1069-779 ka). Different trends of the benthic and planktonic oxygen isotopes, SST and IRD records before and after MIS 25 (~940 ka) evidence the large increase in Northern Hemisphere ice-volume, linked to the cyclicity change from the 41-kyr to the 100-kyr that occurred during the Mid-Pleistocene Transition (MPT). Beside longer glacial-interglacial (G-IG) variability, millennial-scale fluctuations were a pervasive feature across our study. Negative excursions in the benthic d18O time series observed at the times of IRD events may be related to glacio-eustatic changes due to ice sheets retreats and/or to changes in deep hydrography. Time series analysis on surface water proxies (IRD, SST and planktonic d18O) of the interval between MIS 31 to MIS 26 shows that the timing of these millennial-scale climate changes are related to half-precessional (10 kyr) components of the insolation forcing, which are interpreted as cross-equatorial heat transport toward high latitudes during both equinox insolation maxima at the equator.

Stable isotope ratios and foraminiferal abundance of sediment cores from the Western Iberian Margin

Rapid climate changes at the onset of the last deglaciation and during Heinrich Event H4 were studied in detail at IMAGES cores MD95-2039 and MD95-2040 from the Western Iberian margin. A major reorganisation of surface water hydrography, benthic foraminiferal community structure, and deepwater isotopic composition commenced already 540 years before the Last Isotopic Maximum (LIM) at 17.43 cal. ka and within 670 years affected all environments. Changes were initiated by meltwater spill in the Nordic Seas and northern North Atlantic that commenced 100 years before concomitant changes were felt off western Iberia. Benthic foraminiferal associations record the drawdown of deepwater oxygenation during meltwater and subsequent Heinrich Events H1 and H4 with a bloom of dysoxic species. At a water depth of 3380 m, benthic oxygen isotopes depict the influence of brines from sea ice formation during ice-rafting pulses and meltwater spill. The brines conceivably were a source of ventilation and provided oxygen to the deeper water masses. Some if not most of the lower deep water came from the South Atlantic. Benthic foraminiferal assemblages display a multi-centennial, approximately 300-year periodicity of oxygen supply at 2470-m water depth. This pattern suggests a probable influence of atmospheric oscillations on the thermohaline convection with frequencies similar to Holocene climate variations. For Heinrich Events H1 and H4, response times of surface water properties off western Iberia to meltwater injection to the Nordic Seas were extremely short, in the range of a few decades only. The ensuing reduction of deepwater ventilation commenced within 500-600 years after the first onset of meltwater spill. These fast temporal responses lend credence to numerical simulations that indicate ocean-climate responses on similar and even faster time scales.

Pliocene stable oxygen and carbon isotope record of benthic foraminifera from ODP Site 138-846 in the eastern equatorial Pacific

Oxygen and carbon isotope ratios were measured in benthic foraminifers from the entire Pliocene and latest Miocene sections of Site 846, a 180-m section, at a sampling interval of 10 cm. This provides a temporal resolution of about 2500 yr. The documented continuity of the record is excellent. Using the time scale that was developed on the basis of orbital tuning of GRAPE density records, we observed a fairly constant phase relationship between delta18O and variations in the obliquity of Earth's rotational axis. A new numbering scheme for Pliocene isotope stages is proposed. This high-resolution delta18O record clarifies several interesting aspects of late Neogene climatic evolution, including a "glacial" event that may have caused the final Messinian desiccation of the Mediterranean Sea; one or more "interglacial" events that might have caused refilling of the Mediterranean; a well-resolved couplet of glacial events at about the age of the Sidujfall Subchron; interglacial extremes in the early part of the Gauss that could have resulted from either significant deglaciation on Antarctica or from warming of deep water; and a gradual ramp of increasingly extreme "glacial" events, starting at about the Kaena Subchron and culminating with delta18O stage 100 in the earliest Matuyama.

Stable isotope ratios of benthic foraminifera in mid-Pleitsocene sediments of ODP Site 108-664 in the equatorial Atlantic

Five delta13C records from the deep ocean, extending back to 1.3 Ma, were examined in order to constrain changes in mean ocean carbon isotope composition and thermohaline circulation over the 41- to 100-ka climate transition. These data show that significant perturbations in mean ocean carbon chemistry were associated with the mid-Pleistocene climate transition. Notable features of the last 1.3 Myr are (1) a pronounced ~0.3? decrease in mean ocean delta13C between 0.9 and 1.0 Myr, followed by a return to pre-1.0 Ma values by 400 ka B.P., which we propose was due to the onetime addition of isotopically depleted terrestrial carbon to the ocean, possibly associated with an increase in global aridity (and decrease in the size of the biosphere) across the 41- to 100-ka transition; (2) no change in the Atlantic-Pacific (A-P) delta13C gradient over the last 1.3 Myr, suggesting no change in mean ocean nutrient content accompanied the addition of light carbon; and (3) stronger vertical nutrient fractionation in the North Atlantic in the middle Pleistocene between sites 607 and 552, suggesting weaker North Atlantic Deep Water formation at this time relative to the early and late Pleistocene. We also find evidence for a more pronounced deep recirculation gyre in the western North Atlantic basin in the early Brunhes, as evidenced by "aging" of deep northern basin water (site 607) relative to deep water in the equatorial Atlantic (site 664).

Age model, stable isotopes, major element proportions and endmember unmixing results from four western tropical Atlantic cores

We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12° N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9° N) and of Amazon material ("%-Amazon", at 12° N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial periods. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. Increased Amazonian rainfall reflects the intensification of the South American monsoon in response to enhanced land-ocean thermal gradient and moisture convergence. However, low %-Amazon values obtained at 12° N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganizations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume.

Stable carbon and oxygen isotope record of Neogloboquadrina pachyderma and Cibicides lobatulus from the southwestern Greenland Sea

A core transect across the southwestern Greenland Sea reveals coeval events of extremely negative planktic and benthic delta13C excursions between 40 and 87 ka. The most pronounced event, event 1, began at peak Dansgaard-Oeschger stadial 22 (85 ka) with a duration of 18 k.y. During this episode, incursions of Atlantic Intermediate Water caused a bottom-water warming of up to 8 °C. The amplitude, timing, and geographic pattern of the delta13C events suggest that this bottom-water warming triggered clathrate instability along the East Greenland slope and a methane-induced depletion of delta13CDIC (DIC- dissolved inorganic carbon). Since delta13C event 1 matches a major peak in atmospheric CH4 concentration, this clathrate destabilization may have contributed to the rise in atmospheric CH4 and thus to climate warming over marine isotope stage 5.1.

Stable carbon and oxygen isotope ratios of foraminifera from Holocene sediments of the North Atlantic

The Denmark Strait Overflow (DSO) today compensates for the northward flowing Norwegian and Irminger branches of the North Atlantic Current that drive the Nordic heat pump. During the Last Glacial Maximum (LGM), ice sheets constricted the Denmark Strait aperture in addition to ice eustatic/isostatic effects which reduced its depth (today ~630 m) by ~130 m. These factors, combined with a reduced north-south density gradient of the water-masses, are expected to have restricted or even reversed the LGM DSO intensity. To better constrain these boundary conditions, we present a first reconstruction of the glacial DSO, using four new and four published epibenthic and planktic stable-isotope records from sites to the north and south of the Denmark Strait. The spatial and temporal distribution of epibenthic delta18O and delta13C maxima reveals a north-south density gradient at intermediate water depths from sigma0 ~28.7 to 28.4/28.1 and suggests that dense and highly ventilated water was convected in the Nordic Seas during the LGM. However, extremely high epibenthic delta13C values on top of the Mid-Atlantic Ridge document a further convection cell of Glacial North Atlantic Intermediate Water to the south of Iceland, which, however, was marked by much lower density (sigma0 ~28.1). The north-south gradient of water density possibly implied that the glacial DSO was directed to the south like today and fed Glacial North Atlantic Deep Water that has underthrusted the Glacial North Atlantic Intermediate Water in the Irminger Basin.

Stable oxygen isotope, UK'37, productivity, CaCO3 and XRF records from sediment core MD06-3075

Proxy records of hydrologic variability in the West Pacific Warm Pool (WPWP) have revealed wide-scale changes in past convective activity in response to orbital and sub-orbital climate forcings. However, attributing proxy responses to regional changes in WPWP hydrology versus local variations in precipitation requires independent records linking the terrestrial and marine realms. We present high-resolution stable isotope, UK'37 sea-surface temperature, X-ray fluorescence (XRF) core scanning and coccolithophore-derived paleoproductivity records covering the past 120 ka from International Marine Global Change (IMAGES) Program Core MD06-3075 (6°29' N, 125°50' E, water depth 1878 m), situated in the Davao Gulf on the southern side of Mindanao. XRF-derived log(Fe/Ca) records provide a robust proxy for runoff-driven sedimentary discharge from Mindanao, whilst past changes in local productivity are associated with variable freshwater runoff and stratification of the surface layer. Significant precessional-scale variability in sedimentary discharge occurred during Marine Isotope Stage (MIS) 5, with peaks in discharge contemporaneous with Northern Hemisphere summer insolation minima. We attribute these changes to the latitudinal migration of the Intertropical Convergence Zone (ITCZ) over the WPWP together with variability in the strength of the Walker circulation acting on precessional timescales. Between 60 and 15 ka sedimentary discharge at Mindanao was muted, displaying little orbital- or millennial-scale variability, likely in response to weakened precessional insolation forcing and lower sea level driving increased subsidence of air masses over the exposed Sunda Shelf. These results highlight the high degree of local variability in the precipitation response to past climate changes in the WPWP.