Stable isotope record of the Okhotsk Sea

Stable isotope data on benthic foraminifera from more than 30 cores on the northern Emperor Seamounts and in the Okhotsk Sea are synthesized in paleohydrographic profiles spanning the depth range 1000-4000 m. Holocene (core-top) benthic foraminiferal d18O and d13C data are calibrated to modern hydrographic properties through measurements of d13C of SumCO2 and d18O of seawater. Cibicidoides stable isotope ratios are close to the d13C and equilibrium d18O of seawater, whereas Uvigerina d18O and d13C are variably offset from Cibicidoides. Glacial maximum d13C of Cibicidoides displays a different vertical profile than that of the Holocene. When results are adjusted by +0.32 per mil to account for the secular change in d13C during the last glacial maximum, the data coincide with the modern seawater and foraminiferal curves deeper than ~2 km. However, at shallower depths d13C gradually increases by as much as 1 per mil above the modern value. Furthermore, above 2 km the benthic d18O decreases by ~0.5 per mil. These results are consistent with a benthic front at ~2 km in the North Pacific (see Herguera et al., 1992), but they differ from interpretations based on trace metal data which indicate a source of nutrient-depleted deep water during glaciation. The isotopic data suggest that during glaciation there was a better ventilated watermass at intermediate depths in the far northwestern Pacific, it was relatively fresher than deep waters there, and deep waters were as nutrient-rich as today.

Stable Water Isotopologue data collected during HCCT-2010

Cloud samples for the isotopic analysis were collected in the framework of the Hill Cap Cloud Thuringia 2010 (HCCT-2010) campaign on Schmücke (50° 39'N/ 10° 46'E, 937 m a.s.l.; Germany) in September and October 2010 with a three-stage Caltech Active Strand Cloudwater Collector (CASCC) during 13 different cloud events with a temporal resolution of 1 to 3 hours. In a first step, we ensured that no additional fractionation occurred during sampling with the CASCC. The d values of the three sizes classes of the CASCC (4 µm to 16 µm, 16 µm to 22 µm and >22 µm) did not differ significantly, revealing that the cloud droplets of different sizes quickly equilibrate their delta value with the one of the surrounding vapor. delta values in the cloud droplets varied from -77 per mil to -15 per mil in d2H and from -12.1 per mil to -3.9 per mil in d18O and were fitted by d2H =7.8*d18O +13*10**-3. delta values decreased with temperature as well as towards the end of the campaign, representing a seasonal trend which is known from d values in precipitation. The deuterium excess of the cloud samples was generally higher than the Local Meteoric Water Line of the closest GNIP (Global Network of Isotopes in Precipitation) station. Rain decreases its deuterium excess during falling through an unsaturated air column, while the cloud droplets conserve the deuterium excess of the initial evaporation and thus have been found to be a good indicator for the airmass source region: higher deuterium excess was measured for polar air masses and lower deuterium excess for Mediterranean air masses. Changes in d values during one cloud event were up to 3.6 per mil (d2H) and 0.23 per mil (d18O), except for frontal passages, which were associated with increases of ~6 per mil per hour (d2H) and ~0.6 per mil per hour (d18O). Using a box model, we showed that the influence of condensation only was able to explain the variation in the isotope signal of two cloud passages. Consequently, we deduced that the water vapor "feeding" the cloud advected the measured changes. A trajectory analysis and moisture source diagnostic revealed that it is very likely that the variations were either related to rain out along the trajectories or to meteorological changes in the moisture source region. This was the first study using stable water isotopologues in cloud water manifesting their potential in the context of atmospheric water vapor circulation.

Chemical and isotope compositions of thermal and surficial waters from the Chukotka Peninsula

Twenty three groups of thermomineral springs in the eastern Chukotka with discharge temperature from 2 to 97°C and mineralization from 1.47 to 37.14 g/l are studied and compared with surface freshwater from their localities. dD and d18O values in surface waters vary from -121.4 to -89.5 per mil and from -16.4 to -11.1 per mil, respectively, while respective values in thermomineral waters range from -134.2 to -92.5 per mil and from -17.6 to -10.5 per mil. dD value in surface waters decreases from the east to the west, i.e. toward interior areas of the peninsula. Hydrothermal springs most depleted in deuterium (dD < -120 per mil) are localized in the geodynamically active Kolyuchinskaya-Mechigmen Depression. According to the proposed formation model of Chukotka thermomineral waters their observed chemical and isotopic characteristics could result from mixing (in different proportions) of surface waters with the deep-sourced isotopically light mineralized component (dD = ca. -138 per mil, d18O = ca. -19 per mil, ? = from 9.5 to 14.7 g/l). The latter originates most likely from subpermafrost waters subjected to slight cryogenic metamorphism.

Ba/Ca ratios in Neogloboquadrina pachyderma of sedimet core 94B17

Down-core samples of the planktonic foraminifer Neogloboquadrina pachyderma sinistral from the Mendeleyev Ridge in the western Arctic Ocean have been analyzed for Ba/Ca and d18O. The apparent distribution coefficient for N. pachyderma sin. is estimated at DBa = 0.22 ± 0.02. A meltwater event is identified at around 11.8 14C kyr BP and is coincident with elevated Ba/Ca ratios. The barium enrichment is believed to be the result of enhanced weathering and erosion following glaciation. Additionally, barium may have desorbed from shelf sediments as sea level rose. Changes in Ba/Ca correlate with the retreat of the Laurentide Ice Sheet and the evolution of the Mackenzie River drainage basin. Therefore maximum Ba/Ca in Arctic surface waters at 11.8 ka may be indicative of an increase in the export of freshwater from the Arctic to the North Atlantic, potentially contributing to the onset of the Younger Dryas. This work suggests that Ba/Ca in planktonic foraminifera may be a useful indicator of the timing and processes associated with deglaciation.

Sea surafe temperture calculation for the Arabian Sea

Sea surface temperature (SST) and seawater d18O (d18Ow) were reconstructed in a suite of sediment cores from throughout the Arabian Sea for four distinct time intervals (0 ka, 8 ka, 15 ka, and 20 ka) with the aim of understanding the history of the Indian Monsoon and the climate of the Arabian Sea region. This was accomplished through the use of paired Mg/Ca and d18O measurements of the planktonic foraminifer Globigerinoides ruber. By analyzing basin-wide changes and changes in cross-basinal gradients, we assess both monsoonal and regional-scale climate changes. SST was colder than present for the majority of sites within all three paleotime slices. Furthermore, both the Indian Monsoon and the regional Arabian Sea mean climate have varied substantially over the past 20 kyr. The 20 ka and 15 ka time slices exhibit average negative temperature anomalies of 2.5°-3.5°C attributable, in part, to the influences of glacial atmospheric CO2 concentrations and large continental ice sheets. The elimination of the cross-basinal SST gradient during these two time slices likely reflects a decrease in summer monsoon and an increase in winter monsoon strength. Changes in d18Ow that are smaller than the d18O signal due to global ice volume reflect decreased evaporation and increased winter monsoon mixing. SSTs throughout the Arabian Sea were still cooler than present by an average of 1.4°C in the 8 ka time slice. These cool SSTs, along with lower d18Ow throughout the basin, are attributed to stronger than modern summer and winter monsoons and increased runoff and precipitation. The results of this study underscore the importance of taking a spatial approach to the reconstruction of processes such as monsoon upwelling.

Stable oxygen isotope record of corales from the Red Sea

In order to assess the ability of Porites corals to accurately record environmental variations, high-resolution (weekly/biweekly) coral delta18O records were obtained from four coral colonies from the northern Gulf of Aqaba, which grew at depths of 7, 19, 29, and 42 m along one transect. Adjacent to each colony, hourly temperatures, biweekly salinities, and monthly delta18O of seawater were continuously recorded over a period of 14 months (April 1999 to June 2000). Contrary to water temperature, which shows a regular and strong seasonal variation and change with depth, seawater delta18O exhibits a weak seasonality and little change with depth. Positive correlations between seawater delta18O and salinity were observed. The two parameters were related to each other by the equation delta18O Seawater (per mil, VSMOW) = 0.281 * Salinity - 9.14. The high-resolution coral delta18O records from this study show a regular pattern of seasonality and are able to capture fine details of the weekly average temperature records. They resolve more than 95% of the weekly average temperature range. On the other hand, attenuation and amplification of coral seasonal amplitudes were recorded in deep, slow-growing corals, which were not related to environmental effects (temperature and/or seawater delta18O) or sampling resolution. We propose that these result from a combined effect of subannual variations in extension rate and variable rates of spine thickening of skeletal structures within the tissue layer. However, no smoothing or distortion of the isotopic signals was observed due to calcification within the tissue layer in shallow-water, fast-growing corals. The calculations from coral delta18O calibrations against the in situ measurements show that temperature (T) is related to coral delta18O (delta c) and seawater delta18O (delta w) by the equation T (°C) = -5.38 (delta c - delta w) -1.08. Our results demonstrate that coral delta18O from the northern Gulf of Aqaba is a reliable recorder of temperature variations, and that there is a minor contribution of seawater delta18O to this proxy, which could be ignored.

First results from stable-istope measurements on ice cores from Dronning Maud Land

The European Project for Ice Coring in Antarctica (EPICA) focuses on the drilling of two deep ice cores, the first at Dome C and the second at Kohnen station (75°00' S, 0°04' E) in Dronning Maud Land (DML). This paper deals with stable-isotope records from ice cores drilled in DML. In the first season, the deep EPICA DML core reached a depth of 450 m, recovering ice approximately 7000 years old. Generally, the d18O record indicates a stable Holocene climate and shows low variability. However, during the last 4000 years (based on a preliminary time-scale) the d18O values decrease continuously by about 0.6%, and the deuterium excess values increase by about 0.5%. The correlation between d18O and the deuterium excess d is investigated for a 50m long core section and the near-surface snow. High-pass filtered profiles are positively correlated, whereas the correlation between low-pass filtered profiles is negative. A post-depositional effect due to diffusion processes can be seen in a sub-annually resolved profile from snow-pit samples. Changes in the seasonality of the evolution of the snow cover and the consequences for stable-isotope content are demonstrated with data from ice core B31.

Chemical and isotopic composition of interstitial waters and hydrocarbon composition at ODP Site 184-1146

Bottom-simulating reflectors were observed beneath the southeastern slope of the Dongsha Islands in the South China Sea, raising the potential for the presence of gas hydrate in the area. We have analyzed the chemical and isotopic compositions of interstitial water, headspace gas, and authigenic siderite concretions from Site 1146. Geochemical anomalies, including a slight decrease of chlorine concentration in interstitial water, substantial increase of methane concentration in headspace gas, and 18O enrichment in the authigenic siderite concretion below 400 meters below seafloor are probably caused by the decomposition of gas hydrate. The low-chlorine pore fluids contain higher molecular-weight hydrocarbons and probably migrate to Site 1146 along faults or bedded planes.

Hydrogen and oxygen stable isotope analyses of precipitation sampled at Erlangen (southern Germany) between 2010 and 2013

Shallow groundwater aquifers are often influenced by anthropogenic contaminants or increased nutrient levels. In contrast, deeper aquifers hold potentially pristine paleo-waters that are not influenced by modern recharge. They thus represent important water resources, but their recharge history is often unknown. In this study groundwater from two aquifers in southern Germany were analyzed for their hydrogen and oxygen stable isotope compositions. One sampling campaign targeted the upper aquifer that is actively recharged by modern precipitation, whereas the second campaign sampled the confined, deep Benkersandstein aquifer. The groundwater samples from both aquifers were compared to the local meteoric water line to investigate sources and conditions of groundwater recharge. In addition, the deep groundwater was dated by tritium and radiocarbon analyses. Stable and radiogenic isotope data indicate that the deep-aquifer groundwater was not part of the hydrological water cycle in the recent human history. The results show that the groundwater is older than ~20,000 years and most likely originates from isotopically depleted melt waters of the Pleistocene ice age. Today, the use of this aquifer is strictly regulated to preserve the pristine water. Clear identification of such non-renewable paleo-waters by means of isotope geochemistry will help local water authorities to enact and justify measures for conservation of these valuable resources for future generations in the context of a sustainable water management.

Oxygen isotope and Mg/Ca record of planktonic foraminifera of sediment core MD01-2390

Changes in the local freshwater budget over the last 22,000 years have been estimated from a sediment core located in the southern South China Sea (SCS) using a combined approach of Mg/Ca and oxygen isotopes on the planktonic foraminifera Globigerinoides ruber (white) sensu stricto (s.s.). Core MD01-2390 (06°28,12N, 113°24,56E; water depth 1591 m) is located near the glacial paleo-river mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers that drained the exposed Sunda Shelf. The delta18Oseawater record reveals lower average values (-0.96±0.18 per mil) during the Last Glacial Maximum (LGM) when compared with modern values (-0.54±0.18 per mil). Low salinity during the LGM is interpreted to reflect a higher freshwater contribution due to a greater proximity of the core site to the mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers at that time. A general deglacial increasing trend in salinity due to the progressive landward displacement of the coastline during deglacial shelf flooding is punctuated by several short-term shifts towards higher and lower salinity that are likely related to abrupt changes in the intensity of the East Asian summer monsoon. Thus, the deglacial delta18Oseawater changes reflect the combined effects of sea-level-induced environmental changes on the shelf (e.g. phases of retreat and breakdown of the shelf drainage systems) and East Asian monsoon climate change. Lower salinity than at present during the Early Holocene may be attributed to an increase in summer monsoonal precipitation that is corroborated by previous marine and terrestrial studies that report a Preboreal-Early Holocene monsoon optimum in the Asian monsoon region.

Geochemistry of karst headwaters

This study investigated CO2 degassing and related carbon isotope fractionation effects in the Wiesent River that drains a catchment in the karst terrain of the Fraconian Alb, Southern Germany. The river was investigated by physico-chemical and stable isotope analyses of water and dissolved inorganic carbon during all seasons in 2010 along 65 km long downstream transects between source and mouth. This data set contains the results of field and solute parameters (temperature, conductivity, pH, total alkalinity, total CO2, and pCO2) and stable isotope analyses (d2H-H2O, d18O-H2O, d13C-DIC) for the Wiesent River and major tributaries.

Mg/Ca and stable oxygen measurement of the past 27 Ma of ODP Hole 120-747A

We explore the applicability of paired Mg/Ca and 18O/16O measurements on benthic foraminifera from Southern Ocean site 747 to paleoceanographic reconstructions on pre-Pleistocene timescales. We focus on the late Oligocene through Pleistocene (27-0 Ma) history of paleotemperatures and the evolution of the d18O values of seawater (d18Osw) at a temporal resolution of ~100-200 kyr. Absolute paleotemperature estimates depend on assumptions of how Mg/Ca ratios of seawater have changed over the past 27 Myr, but relative changes that occur on geologically brief timescales are robust. Results indicate that at the Oligocene to Miocene boundary (23.8 Ma), temperatures lag the increase in global ice-volume deduced from benthic foraminiferal d18O values, but the smaller-scale Miocene glaciations are accompanied by ocean cooling of -1°C. During the mid-Miocene phase of Antarctic ice sheet growth (~15-13 Ma), water temperatures cool by ~3°C. Unlike the benthic foraminiferal d18O values, which remain relatively constant thereafter, temperatures vary (by 3°C) and reach maxima at ~12 and ~8.5 Ma. The onset of significant Northern Hemisphere glaciation during the late Pliocene is synchronous with an ~4°C cooling at site 747. A comparison of our d18Osw curve to the Haq et al. (1987, doi:10.1126/science.235.4793.1156 ) sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18O/16O ratios. At ~12-11 Ma in particular, when benthic foraminiferal d18O values do not support a further increase in ice volume, the d18Osw curve comes to a maximum that corresponds to a major mid-Miocene sea level regression. The agreement between the character of our Mg/Ca-based d18Osw curve and sequence stratigraphy demonstrates that benthic foramaniferal Mg/Ca ratios can be used to trace the d18Osw on pre-Pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater Mg/Ca ratios. The Mg/Ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes, which can lead to misinterpretations of the d18O record.

Diagenetic dolomite formation in Quaternary anoxic diatomaceous muds at DSDP Leg 64 Holes

During drilling in the Gulf of California, diagenetic carbonate rocks were recovered at 7 out of 8 sites. These are primarily dolomites which record 13C isotopic evidence of the incorporation of carbon derived from the decomposition of organic matter. In Hole 479, drilled to a sub-bottom depth of 440 meters on the Guaymas Slope, under a fertile upwelling belt, we recognized an excellent example of deep sea dolomitization in progress. This Quaternary section of organic-carbon- rich, low-carbonate, hemipelagic diatomaceous oozes contains numerous fine-grained, decimeter-thin, episodic beds of dolomite, which show sedimentologic, geochemical, and isotopic evidence of accretion by precipitation below 40 meters sub-bottom in zones of high alkalinity and low sulfate. The beds preserve original sedimentary structures. Carbon-13 varies from +3 to +14 per mil, indicating biogenic CO2 reservoirs related to active methanogenesis. In single beds, 18O values range outwardly from +5 to -7 per mil, reflecting increasing temperature with progressive accretion of dolomite with depth; the values parallel progressive trends in lithification, texture, mineralogy, and fossil preservation. We estimate slow accretion rates on the order of 0.1-0.7 mm/10**3 yr. with burial. Dolomitization does not proceed merely at the expense of nearby nannofossils. Ca and Mg ions must be derived from interstitial waters. The episodic appearance of beds in the sequence seems partly a reflection of latent climate signals. This process of deep sea dolomitization carries implications for hydrocarbon migration, as well as an interpretation of the presence of dolomite in other modern and ancient pelagic to hemipelagic sediment sequences.

Interstitial water chemistry at DSDP Leg 67 Holes

Interstitial water chemistry has proved to be a sensitive indicator for early diagenetic reactions, particularly those related to organic matter oxidation. Downhole chemical variations in the pore waters from Deep Sea Drilling Project Holes 496 and 497 on the Middle America Trench slope off Guatemala are anomalous because both salinity and chlorinity show strong decreases to half the values of seawater, and d18O values become positive (maximum of about +2.5% at the bottom of the holes). These observations are explained in terms of dilution of pore waters after retrieval as a result of decomposition of the gas hydrates before removal of pore waters by shipboard squeezing techniques. In all holes, except Hole 495 (drilled in pelagic sediments), decomposition of organic matter leads to rapid sulfate depletion and subsequent methane generation. Associated with methane generation are large increases in alkalinity and dissolved ammonia. The latter component causes ion exchange reactions with clay minerals, which results in maxima in magnesium and perhaps potassium. At greater depths, as yet unidentified reactions cause the removal of magnesium. Especially in the deeper Trench Sites 499 and 500, rapid variations in calcium, magnesium, and alkalinity occur in turbidite sequences.

Isotopic composition of ground ice, ebullition gases and thermokarst lake water, North America

Thermokarst lakes are thought to have been an important source of methane (CH4) during the last deglaciation when atmospheric CH4 concentrations increased rapidly. Here we demonstrate that meltwater from permafrost ice serves as an H source to CH4 production in thermokarst lakes, allowing for region-specific reconstructions of dD-CH4 emissions from Siberian and North American lakes. dD CH4 reflects regionally varying dD values of precipitation incorporated into ground ice at the time of its formation. Late Pleistocene-aged permafrost ground ice was the dominant H source to CH4 production in primary thermokarst lakes, whereas Holocene-aged permafrost ground ice contributed H to CH4 production in later generation lakes. We found that Alaskan thermokarst lake dD-CH4 was higher (-334 ± 17 per mil) than Siberian lake dD-CH4 (-381 ± 18 per mil). Weighted mean dD CH4 values for Beringian lakes ranged from -385 per mil to -382 per mil over the deglacial period. Bottom-up estimates suggest that Beringian thermokarst lakes contributed 15 ± 4 Tg CH4 /yr to the atmosphere during the Younger Dryas and 25 ± 5 Tg CH4 /yr during the Preboreal period. These estimates are supported by independent, top-down isotope mass balance calculations based on ice core dD-CH4 and d13C-CH4 records. Both approaches suggest that thermokarst lakes and boreal wetlands together were important sources of deglacial CH4.