Mode transitions in Northern Hemisphere glaciation: co-evolution of millennial and orbital variability in Quaternary climate

We present a 3.2 Myr record of stable isotopes and physical properties at IODP Site U1308 (reoccupation of DSDP Site 609) located within the ice-rafted detritus (IRD) belt of the North Atlantic. We compare the isotope and lithological proxies at Site U1308 with other North Atlantic records (e.g., sites 982, 607/U1313, and U1304) to reconstruct the history of orbital and millennial-scale climate variability during the Quaternary. The Site U1308 record documents a progressive increase in the intensity of Northern Hemisphere glacial–interglacial cycles during the late Pliocene and Quaternary, with mode transitions at  ∼  2.7, 1.5, 0.9, and 0.65 Ma. These transitions mark times of change in the growth and stability of Northern Hemisphere ice sheets. They also coincide with increases in vertical carbon isotope gradients between the intermediate and deep ocean, suggesting changes in deep carbon storage and atmospheric CO2. Orbital and millennial climate variability co-evolved during the Quaternary such that the trend towards larger and thicker ice sheets was accompanied by changes in the style, frequency, and intensity of millennial-scale variability. This co-evolution may be important for explaining the observed patterns of Quaternary climate change

Rhizon sampler alteration of deep ocean sediment interstitial water samples, as indicated by chloride concentration and oxygen and hydrogen isotopes

Despite their potential to inform past ocean salinity, δ^(18)O, and temperature, high-resolution depth profiles of interstitial water chloride concentration and hydrogen and oxygen isotopes exist in very few locations. One of the primary limitations to the recovery of these depth profiles is that traditional interstitial water sampling requires 5–10 cm whole rounds of the sediment core, which has the potential to interfere with stratigraphic continuity. The Rhizon sampler, a nondestructive tool developed for terrestrial sediment interstitial water extraction, has been proposed for efficient and nondestructive sampling of ocean sediment pore waters. However, there exists little documentation on the reliability and performance of Rhizon samplers in deep ocean sediments, particularly in regard to their effect on chloride concentration and oxygen and hydrogen isotopic measurements. We perform an intercomparison of chloride concentration and oxygen and hydrogen isotopic composition in samples taken using traditional squeezing versus those taken with Rhizon samplers. We find that samples taken with Rhizons have positive biases in both chloride concentration and stable isotopic ratios relative to those taken by squeezing water from sediments in a hydraulic press. The measured offsets between Rhizon and squeeze samples are consistent with a combination of absorption by and diffusive fractionation through the hydrophilic membrane of the Rhizon sampler. These results suggest caution is needed when using Rhizons for sampling interstitial waters in any research of processes that leave a small signal-to-noise ratio in dissolved concentrations or isotope ratios

Precise and accurate isotope fractionation factors (α17O, α18O and αD) for water and CaSO4·2H2O (gypsum)

Gypsum (CaSO4·2H2O) is a hydrated mineral containing crystallization water, also known as gypsum hydration water (GHW). We determined isotope fractionation factors (α17O, α18O and αD) between GHW and free water of the mother solution in the temperature range from 3 °C to 55 °C at different salinities and precipitation rates. The hydrogen isotope fractionation factor (αDgypsum-water) increases by 0.0001 units per °C between 3 °C and 55 °C and salinities <150 g/L of NaCl. The αDgypsum-water is 0.9812 ± 0.0007 at 20 °C, which is in good agreement with previous estimates of 0.981 ± 0.001 at the same temperature. The α18Ogypsum-water slightly decreases with temperature by 0.00001 per °C, which is not significant over much of the temperature range considered for paleoclimate applications. Between 3 °C and 55 °C, α18Ogypsum-water averages 1.0035 ± 0.0002. This value is more precise than that reported previously (e.g. 1.0041 ± 0.0004 at 25 °C) and lower than the commonly accepted value of 1.004. We found that NaCl concentrations below 150 g/L do not significantly affect α18Ogypsum-water, but αDgypsum-water increases linearly with NaCl concentrations even at relatively low salinities, suggesting a salt correction is necessary for gypsum formed from brines. Unlike oxygen isotopes, the αDgypsum-water is affected by kinetic effects that increase with gypsum precipitation rate. As expected, the relationship of the fractionation factors for 17O and 18O follows the theoretical mass-dependent fractionation on Earth (θ = 0.529 ± 0.001). We provide specific examples of the importance of using the revised fractionation factors when calculating the isotopic composition of the fluids

High-resolution pleistocene diatom biostratigraphy of site 983 and correlations with isotope stratigraphy

International audienceHigh accumulation rates and the presence of well-preserved, abundant diatoms in Site 983 sediments from the Gardar Drift gave us the opportunity to refine the Pleistocene diatom biostratigraphic resolution of the high-latitude North Atlantic. Eight Pleistocene diatom datum events are identified and, for the first time, tied directly to the oxygen isotope record and paleomagnetic stratigraphy of Site 983. These datum events are (1) the last occurrence (LO) of Proboscia curvirostris at 0.3 Ma, (2) the LO of Thalassiosira jouseae at 0.3 Ma, (3) the LO of Nitzschia reinholdii at 0.6 Ma, (4) the LO of Nitzschia fossilis at 0.68 Ma, (5) the LO of Nitzschia seminae at 0.84 Ma, (6) the first occurrence (FO) of N. seminae at 1.25 Ma, (7) the FO of Proboscia curvirostris at 1.53 Ma, and (8) the FO of Pseudoeunotia doliolus at 1.89 Ma. Most of these datums are found to be synchronous between the middle and high latitudes of the North Atlantic and the North Pacific. On the basis of these datums, four highlatitude North Atlantic diatom zones are proposed for the Pleistocene. The record of diatom abundance and preservation at Site 983 gives evidence for the influence of fluctuating Pleistocene climatic conditions on diatom productivity in the high-latitude North Atlantic

Abrupt weakening of the summer monsoon in northwest India ~4100 yr ago

Climate change has been suggested as a possible cause for the decline of urban centers of the Indus Civilization ∼4000 yr ago, but extant paleoclimatic evidence has been derived from locations well outside the distribution of Indus settlements. Here we report an oxygen isotope record of gastropod aragonite (δ18Oa) from Holocene sediments of paleolake Kotla Dahar (Haryana, India), which is adjacent to Indus settlements and documents Indian summer monsoon (ISM) variability for the past 6.5 k.y. A 4‰ increase in δ18Oa occurred at ca. 4.1 ka marking a peak in the evaporation/precipitation ratio in the lake catchment related to weakening of the ISM. Although dating uncertainty exists in both climate and archaeological records, the drought event 4.1 ka on the northwestern Indian plains is within the radiocarbon age range for the beginning of Indus de-urbanization, suggesting that climate may have played a role in the Indus cultural transformation

IODP Drilling of the "Shackleton Sites" on the Iberian Margin: A Plio-Pleistocene Marine Reference Section of Millennial-Scale Climate Change

Few marine sediment cores have played such a pivotal role in paleoclimate research as those recovered from the Portuguese Margin, including MD95-2039 to MD95-2042 (hereafter referred to as the “Shackleton sites”) (Fig. 1). These cores preserve a high-fidelity record of millennial-scale climate variability for the last several glacial cycles and can be readily correlated to Greenland ice cores. Moreover, the narrow continental shelf and proximity of the Tagus River results in rapid delivery of terrestrial material to the deep-sea environment, thereby permitting correlation of marine and ice-core records to European terrestrial sequences. Few places exist in the world’s ocean where such marine-ice-terrestrial linkages are possible. Consequently, the Iberian Margin cores have become de facto reference sections for the study of abrupt climate change

Relative paleointensity (RPI) and age control in Quaternary sediment drifts off the Antarctic Peninsula

Lack of foraminiferal carbonate in marine sediments deposited at high latitudes results in traditional oxygen isotope stratigraphy not playing a central role in Quaternary age control for a large portion of the globe. This limitation has affected the interpretation of Quaternary sediment drifts off the Antarctic Peninsula in a region critical for documenting past instability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS). Here we use piston cores recovered from these sediment drifts in 2015 during cruise JR298 of the RRS James Clark Ross to test the usefulness for age control of relative paleointensity (RPI) data augmented by scant δ 18 O data. Thermomagnetic and magnetic hysteresis data, as well as isothermal remanent magnetization (IRM) acquisition curves, indicate the presence of prevalent magnetite and subordinate oxidized magnetite (“maghemite”) in the cored sediments. The magnetite is likely detrital. Maghemite is an authigenic mineral, associated with surface oxidation of magnetite grains, which occurs preferentially in the oxic zone of the uppermost sediments, and buried oxic zones deposited during prior interglacial climate stages. Low concentrations of labile organic matter apparently led to arrested pore-water sulfate reduction explaining oxic zone burial and downcore survival of the reactive maghemite coatings. At some sites, maghemitization has a debilitating effect on RPI proxies whereas at other sites maghemite is less evident and RPI proxies can be adequately matched to the RPI reference template. Published RPI data at ODP Site 1101, located on Drift 4, can be adequately correlated to contemporary RPI templates, probably as a result of disappearance (dissolution) of maghemite at sediment depths >∼10 m

Oxygen and hydrogen isotope signatures of Northeast Atlantic water masses

Only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in NE Atlantic water masses, and data are especially sparse for intermediate and deep-water masses. The current study greatly expands this record with 527 δ18O values from 47 stations located throughout the mid- to low-latitude NE Atlantic. In addition, δD was analyzed in the 192 samples collected along the GEOTRACES North Atlantic Transect GA03 (GA03_e = KN199-4) and the 115 Iberia-Forams cruise samples from the western and southern Iberian margin. An intercomparison study between the two stable isotope measurement techniques (cavity ring-down laser spectroscopy and magnetic-sector isotope ratio mass spectrometry) used to analyze GA03_e samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios. The surface (0-100 m) and central (100-500 m) water isotope data show the typical, evaporation related trend of increasing values equatorward with the exception for the zonal transect off Cape Blanc, NW Africa. Off Cape Blanc, surface water isotope signatures are modified by the upwelling of fresher Antarctic Intermediate Water (AAIW) that generally has isotopic values of 0.0 to 0.5‰ for δ18O and 0 to 2‰ for δD. Along the Iberian margin the Mediterranean Outflow Water (MOW) is clearly distinguished by its high δ18O (0.5-1.1‰) and δD (3-6‰) values that can be traced into the open Atlantic. Isotopic values in the NE Atlantic Deep Water (NEADW) are relatively low (δ18O: –0.1 to 0.5‰; δD: –1 to 4‰) and show a broader range than observed previously in the northern and southern convection areas. The NEADW is best observed at GA03_e Stations 5 and 7 in the central NE Atlantic basin. Antarctic Bottom Water isotope values are relatively high indicating modification of the original Antarctic source water along the flow path. The reconstructed δ18O-salinity relationship for the complete data set has a slope of 0.51, i.e. slightly steeper than the 0.46 described previously by Pierre et al. (1994) for the tropical to subtropical Northeast Atlantic. This slope decreases to 0.46 for the subtropical North Atlantic Central Water (NACW) and the MOW and to 0.32 for the surface waters of the upper 50 m. The δD-salinity mixing lines have estimated slopes of 3.01 for the complete data, 1.26 for the MOW, 3.47 for the NACW, and 2.63 for the surface waters. The slopes of the δ18O-δD relationship are significantly lower than the one for the Global Meteoric Water Line with 5.6 for the complete data set, 2.30 for the MOW, 4.79 for the NACW, and 3.99 for the surface waters. The lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the subtropics.EC Grant agreement n° 228344 - EUROFLEETS, C2007-FCT/319/2006info:eu-repo/semantics/publishedVersio

Rhizon sampler alteration of deep ocean sediment interstitial water samples, as indicated by chloride concentration and oxygen and hydrogen isotopes

Despite their potential to inform past ocean salinity, δ18O, and temperature, high-resolution depth profiles of interstitial water chloride concentration and hydrogen and oxygen isotopes exist in very few locations. One of the primary limitations to the recovery of these depth profiles is that traditional interstitial water sampling requires 5–10 cm whole rounds of the sediment core, which has the potential to interfere with stratigraphic continuity. The Rhizon sampler, a nondestructive tool developed for terrestrial sediment interstitial water extraction, has been proposed for efficient and nondestructive sampling of ocean sediment pore waters. However, there exists little documentation on the reliability and performance of Rhizon samplers in deep ocean sediments, particularly in regard to their effect on chloride concentration and oxygen and hydrogen isotopic measurements. We perform an intercomparison of chloride concentration and oxygen and hydrogen isotopic composition in samples taken using traditional squeezing versus those taken with Rhizon samplers. We find that samples taken with Rhizons have positive biases in both chloride concentration and stable isotopic ratios relative to those taken by squeezing water from sediments in a hydraulic press. The measured offsets between Rhizon and squeeze samples are consistent with a combination of absorption by and diffusive fractionation through the hydrophilic membrane of the Rhizon sampler. These results suggest caution is needed when using Rhizons for sampling interstitial waters in any research of processes that leave a small signal-to-noise ratio in dissolved concentrations or isotope ratios

Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean

We demonstrate that the carbon isotopic signal of mid-depth waters evolved differently from deep waters in the South Atlantic sector of the Southern Ocean during the Pleistocene. Deep sites (>3700 m) exhibit large glacial-to-interglacial variations in benthic d13C, whereas the amplitude of the d13C signal at Site 1088 (2100 m water depth) is small. Unlike the deep sites, at no time during the Pleistocene were benthic d13C values at Site 1088 lower than those of the deep Pacific. Reconstruction of intermediate-todeep d13C gradients (D13CI-D) supports the existence of a sharp chemocline between 2100 and 2700 m during most glacial stages of the last 1.1 myr. This chemical divide in the glacial Southern Ocean separated well-ventilated water above 2500 m from poorly ventilated water below. The D13CI-D signal parallels the Vostok atmospheric pCO2 record for the last 400 kyr, lending support to physical models that invoke changes in Southern Ocean deep water ventilation as a mechanism for changing atmospheric pCO2. The emergence of a strong 100-kyr cycle in D13CI-D during the mid-Pleistocene supports a change in vertical fractionation and deep-water ventilation rates in the Southern Ocean, and is consistent with possible CO2- forcing of this climate transition. Components: 7562 words, 14 figures, 2 tables