Mean air temperatures of Greenland

Melt rate and surface temperature on the Greenland ice sheet are parameterized in terms of snow accumulation, mean annual air temperatur and mean July air temperature. Melt rates are calculated using positive degree-days, and firn warming (i.e. the positive deviation of the temperature at 10-15 m depth from the mean annual air temperature) is estimated from the calculated amount of refrozen melt water in the firn. A comparison between observed and calculated melt rates shows that the parameterization provides a reasonable estimate of the present ablation rates in West Greenland between 61°N and 76°N. The average equilibrium line elevation is estimated to be about 1150 m and 1000 m for West and East Greenland respectively, which is several hundred meter lower than previous estimates. However, the total annual ablation from the ice sheet is found to be about 280 km**3 of water per year which agrees well with most other estimates. The melt-rate model predicts significant melting and consequently significant firn warming even at the highest elevations of the South Greenland ice sheet, whereas a large region of central Greenland north of 70° N experiences little or no summer melting. This agrees with the distribution of the dry snow facics as given by BENSON (1962).

Comparison of accumulation rates and stable isotope ratios

Since 1979/80, glaciological studies have been carried out at Ekströmisen, Antarctica, including accumulation-stake measurements, snow-pit and shallow-firn-core studies. Snowstratigraphy, chemical properties and stable-isotope ratios (d18O) were investigated. This study focuses on three cores taken between 1982 and 1998. The 1998 core was dated using dielectric profiling, d18O profiles and stake measurements. Accumulation rates showhigh interannual and spatial variability due to the extreme wind influence. No significant trend was found for the last 50 years; during the first half of the 20th century, accumulation decreased. The high spatial and interannual variability, however, means that trends must be interpreted with care. In spite of the highly irregular accumulation distribution, stable-isotope ratios show little spatial variability. The mean annual d18O values of cores B04 and FB0198 agree fairly well for the time period 1955-82 covered by both cores. d18O values have increased during most of the 20th century; since the late 1980s a decrease is observed. This change is not related to air temperature, since mean annual air temperatures at Neumayer show no significant trend over the last two decades.

Sample locations, CTD data, mean spring and annual temperatures and salinities from World Ocean Atlas (WOA, 2001), and ICP-OES Mg/Ca data

Due to its strong gradient in salinity and small temperature gradient the Mediterranean provides an ideal setting to study the impact of salinity on the incorporation of Mg into foraminiferal tests. We have investigated tests of Globorotalia inflata and Globigerina bulloides in plankton tow and core top samples from the Western Mediterranean using ICP-OES for bulk analyses and LA-ICP-MS for analyses of individual chambers in single specimens. Mg/Ca observed in G. inflata are consistent with existing calibrations, whereas G. bulloides had significantly higher Mg/Ca than predicted, particularly in core top samples from the easterly stations. Scanning Electron Microscopy and Laser Ablation ICP-MS revealed secondary overgrowths on some tests, which could explain the observed high core top Mg/Ca. We suggest that the Mediterranean intermediate and deep water supersaturated with respect to calcite cause these overgrowths and therefore increased bulk Mg/Ca. However, the different species are influenced by diagenesis to different degrees probably due to different test morphologies. Our results provide new perspectives on reported anomalously high Mg/Ca in sedimentary foraminifera and the applicability of the Mg/Ca paleothermometry in high salinity settings, by showing that (1) part of the signal is generated by precipitation of inorganic calcite on the foraminifer test due to increased calcite saturation state of the water and (2) species with high surface-to-volume shell surfaces are potentially more affected by secondary Mg-rich calcite encrustation.

Curie temperatures and hysteresis properties of ODP Leg 183 sites, Kerguelen Plateau and Broken Ridge

In this manuscript, we present rock magnetic results of samples recovered during Leg 183. The Leg 183 cores were recovered from six drill sites and display variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Cretaceous subaerial basalt samples with titanomagnetite exhibit a strong Verwey transition in the vicinity of 110 K and have frequency-dependent susceptibility curves that resemble those of synthetic (titano) magnetites. These results are in good agreement with the thermomagnetic characteristics where titanomagnetites with Curie temperatures of ~580°C were identified. The hysteresis ratios suggest that the bulk magnetic grain size is in the psuedo-single-domain boundary. These subaerial basalts experienced high-temperature oxidation and maintained reliable paleomagnetic records. In contrast, the 34-Ma submarine pillow basalts do not show the Verwey transition during the low-temperature experiments. Thermomagnetic analysis shows that the remanent magnetization in this group is mainly carried by a thermally unstable mineral titanomaghemite. The frequency-dependent relationships are opposite of those from the first group and show little sign of titanomagnetite characteristics. Rocks from the third group are oxidized titanomagnetites and have multiple magnetic phases. They have irreversible thermaomagnetic curves and hysteresis ratios clustering toward the multidomain region (with higher Hcr/Hc ratios). The combined investigation suggests that variations in magnetic properties correlate with changes in lithology, which results in differences in the abundance and size of magnetic minerals. The rock magnetic data on Leg 183 samples clearly indicate that titanomagnetite is the dominant mineral and the primary remanence carrier in subaerial basalt. The generally good magnetic stability and other properties exhibited by titanomagnetite-bearing rocks support the inference that the ChRM isolated from the Cretaceous sites were acquired during the Cretaceous Normal Superchron. The stable inclinations identified from these samples are therefore useful for future tectonic studies.

Geochemistry, isotopes, trace elements and fluid inclusion geothermometry at DSDP Hole 83-504B

Stockwork-like metal sulfide mineralizations were found at 910-928 m below seafloor (BSF) in the pillow/dike transition zone of Hole 504B. This is the same interval where most physical properties of the 5.9-m.y.-old crust of the Costa Rica Rift change from those characteristic of Layer 2B to those of Layer 2C. The pillow lavas, breccias, and veins of the stockwork-like zone were studied by transmitted and reflected light microscopy, X-ray diffraction, and electron microprobe analysis. Bulk rock oxygen isotopic analyses as well as isolated mineral oxygen and sulfur isotopic analyses and fluid inclusion measurements were carried out. A complex alteration history was reconstructed that includes three generations of fissures, each followed by precipitation of characteristic hydrothermal mineral parageneses: (1) Minor and local deposition of quartz occurred on fissure walls; adjacent wall rocks were silicified, followed by formation of chlorite and minor pyrite I in the veins, whereas albite, sphene, chlorite and chlorite-expandable clay mixtures, actinolite, and pyrite replaced igneous phases in the host rocks. The hydrothermal fluids responsible for this first stage were probably partially reacted seawater, and their temperatures were at least 200-250° C. (2) Fissures filled during the first stage were reopened and new cracks formed. They were filled with quartz, minor chlorite and chlorite-expandable clay mixtures, traces of epidote, common pyrite, sphalerite, chalcopyrite, and minor galena. During the second stage, hydrothermal fluids were relatively evolved metal- and Si-rich solutions whose temperatures ranged from 230 to 340° C. The fluctuating chemical composition and temperature of the solutions produced a complex depositional sequence of sulfides in the veins: chalcopyrite I, ± Fe-rich sphalerite, chalcopyrite II ("disease"), Fe-poor sphalerite, chalcopyrite III, galena, and pyrite II. (3) During the last stage, zeolites and Mg-poor calcite filled up the remaining spaces and newly formed cracks and replaced the host rock plagioclase. Analcite and stilbite were first to form in veins, possibly at temperatures below 200°C; analcite and earlier quartz were replaced by laumontite at 250°C, whereas calcite formation temperature ranged from 135 to 220°C. The last stage hydrothermal fluids were depleted in Mg and enriched in Ca and 18O compared to seawater and contained a mantle carbon component. This complex alteration history paralleling a complex mineral paragenesis can be interpreted as the result of a relatively long-term evolution of a hydrothermal system with superimposed shorter term fluctuations in solution temperature and composition. Hydrothermal activity probably began close to the axis of the Costa Rica Rift with the overall cooling of the system and multiple fracturing stages due to movement of the crust away from the axis and/or cooling of a magmatic heat source.

Dinoflagellate cyst assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes, and geochemical parameters of sediment core GeoB9508-5

In order to investigate a possible connection between tropical northeast (NE) Atlantic primary productivity, Atlantic meridional overturning circulation (AMOC), and drought in the Sahel region during Heinrich Stadial 1 (HS1), we used dinoflagellate cyst (dinocyst) assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes (d13C) and geochemical parameters of a marine sediment core (GeoB 9508-5) from the continental slope offshore Senegal. Our results show a two-phase productivity pattern within HS1 that progressed from an interval of low marine productivity between ~ 19 and 16 kyr BP to a phase with an abrupt and large productivity increase from ~ 16 to 15 kyr BP. The second phase is characterized by distinct heavy planktonic d13C values and high concentrations of heterotrophic dinocysts in addition to a significant cooling signal based on reconstructions of past sea surface temperatures (SST). We conclude that productivity variations within HS1 can be attributed to a substantial shift of West African atmospheric processes. Taken together our results indicate a significant intensification of the North East (NE) trade winds over West Africa leading to more intense upwelling during the last millennium of HS1 between ~ 16 and 15 kyr BP, thus leaving a strong imprint on the dinocyst assemblages and sea surface conditions. Therefore, the two-phase productivity pattern indicates a complex hydrographic setting suggesting that HS1 cannot be regarded as uniform as previously thought.