Gisela Winckler

Gas hydrate destabilization: enhanced dewatering, benthic material turnover and large methane plumes at the Cascadia convergent margin

Mixed methane–sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the Cascadia convergent margin. Vent fields from which methane-charged, low-salinity fluids containing sulfide, ammonia, 4He, and isotopically light CO2 escape are associated with these exposures. They characterize a newly recognized mechanism of dewatering at convergent margins, where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward. This process augments the convergence-generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate. Discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths. The enhanced turnover results from the oxidation of methane, hydrogen sulfide, and ammonia by vent biota. The injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide. A large fraction of the methane is oxidized within the water column and generates δ13C anomalies of the dissolved inorganic carbon pool.

Covariant Glacial-Interglacial Dust Fluxes in the Equatorial Pacific and Antarctica

Dust plays a critical role in Earths climate system and serves as a natural source of iron and other micronutrients to remote regions of the ocean. We have generated records of dust deposition over the past 500,000 years at three sites spanning the breadth of the equatorial Pacific Ocean. Equatorial Pacific dust fluxes are highly correlated with global ice volume and with dust fluxes to Antarctica, which suggests that dust generation in interhemispheric source regions exhibited a common response to climate change over late-Pleistocene glacial cycles. Our results provide quantitative constraints on the variability of aeolian iron supply to the equatorial Pacific Ocean and, more generally, on the potential contribution of dust to past climate change and to related changes in biogeochemical cycles.

Fluid venting in the eastern Aleutian subduction zone

Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over- steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane- or sulfide-dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5 + 0.7 L m -2 d -1 for tectonics-induced water flow ( Wallmann et al., 1997b). A geophysical estimate of dewatering from the same area (von Huene et al., 1997) based on sediment porosity reduction shows a fluid loss of 0.02 L m -2 d-1 for a 5.5 km wide converged segment near the deformation front. Our video-guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m -2 d -1 is estimated from the biogeochemical approach. The two estimates for tectonics-induced water flow from the accretionary prism are in surprisingly good agreement.

Increased Dust Deposition in the Pacific Southern Ocean During Glacial Periods

Dust deposition in the Southern Ocean constitutes a critical modulator of past global climate variability, but how it has varied temporally and geographically is underdetermined. Here, we present data sets of glacial-interglacial dust-supply cycles from the largest Southern Ocean sector, the polar South Pacific, indicating three times higher dust deposition during glacial periods than during interglacials for the past million years. Although the most likely dust source for the South Pacific is Australia and New Zealand, the glacial-interglacial pattern and timing of lithogenic sediment deposition is similar to dust records from Antarctica and the South Atlantic dominated by Patagonian sources. These similarities imply large-scale common climate forcings, such as latitudinal shifts of the southern westerlies and regionally enhanced glaciogenic dust mobilization in New Zealand and Patagonia. A million-year-long marine sedimentary record of dust supply to the Pacific Southern Ocean reflects global climate. Dust in the Sea The effect of windblown dust on marine productivity in the Southern Ocean is thought to be a key determinant of atmospheric CO2 concentrations. Lamy et al. (p. 403) present a record of dust supply to the Pacific sector of the Southern Ocean for the past one million years, derived from a suite of deep-sea sediment cores. Dust deposition during glacial periods was 3 times greater than during interglacials, and its major source region was probably Australia or New Zealand.

Carbon isotopes and habitat of polar planktic foraminifera in the Okhotsk Sea: the ‘carbonate ion effect’ under natural conditions

The upper 500 or 1000 m of the water column in the Okhotsk Sea was sampled for living planktic foraminifera. The polar species Neogloboquadrina pachyderma (sinistral) strongly dominates the foraminiferal assemblage; the subpolar to temperate species Globigerina bulloides accounts for 10–25%. Other species account for up to 3% only. The shell δ18O and δ13C values of the species N. pachyderma (sin.) are compared to water δ18O values and δ13C values of dissolved inorganic carbon (DIC). The strong gradient in δ18O composition and temperature in the upper water column is reflected in the δ18O of N. pachyderma (sin.). Relative to the values expected for inorganic calcite precipitated under equilibrium conditions N. pachyderma (sin.) displays a vital effect of about 1‰ in δ18O. The δ13C composition of N. pachyderma (sin.) is about constant with water depth and the reflection of δ13CDIC in the foraminiferal shell seems to be masked by other effects. Most foraminifera are found above or slightly below the thermocline and can be assumed to calcify in the upper 200 m of the water column. The gradient of δ13CDIC extends well below this depth, therefore the lack of correlation can partly be attributed to this fact. The remaining discrepancy between δ13C of N. pachyderma (sin.) and δ13CDIC correlates with the carbonate ion concentration in the water column. This leads to the conclusion that the ‘carbonate ion effect’ (CIE), which has been derived from culturing experiments for other species [Spero et al. (1997) Nature 390, 497–500], is found here under natural conditions. When the magnitude of the CIE derived for G. bulloides is applied to N. pachyderma (sin.), CIE-corrected δ13C of N. pachyderma (sin.) is a direct reflection of δ13CDIC in the water column with a constant offset of 1.2‰.

Comparing modeled and observed changes in mineral dust transport and deposition to Antarctica between the Last Glacial Maximum and current climates

Mineral dust aerosols represent an active component of the Earth’s climate system, by interacting with radiation directly, and by modifying clouds and biogeochemistry. Mineral dust from polar ice cores over the last million years can be used as paleoclimate proxy, and provide unique information about climate variability, as changes in dust deposition at the core sites can be due to changes in sources, transport and/or deposition locally. Here we present results from a study based on climate model simulations using the Community Climate System Model. The focus of this work is to analyze simulated differences in the dust concentration, size distribution and sources in current climate conditions and during the Last Glacial Maximum at specific ice core locations in Antarctica, and compare with available paleodata. Model results suggest that South America is the most important source for dust deposited in Antarctica in current climate, but Australia is also a major contributor and there is spatial variability in the relative importance of the major dust sources. During the Last Glacial Maximum the dominant source in the model was South America, because of the increased activity of glaciogenic dust sources in Southern Patagonia-Tierra del Fuego and the Southernmost Pampas regions, as well as an increase in transport efficiency southward. Dust emitted from the Southern Hemisphere dust source areas usually follow zonal patterns, but southward flow towards Antarctica is located in specific areas characterized by southward displacement of air masses. Observations and model results consistently suggest a spatially variable shift in dust particle sizes. This is due to a combination of relatively reduced en route wet removal favouring a generalized shift towards smaller particles, and on the other hand to an enhanced relative contribution of dry coarse particle deposition in the Last Glacial Maximum.

HYDROTHERMAL GASES OFFSHORE MILOS ISLAND, GREECE

Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Milos. The gases in these fluids contain mostly CO2 but CH4 concentrations up to 2% are present. The stable carbon isotopic composition of the CO2 (near 0%) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH4 samples are enriched in the heavy species ([delta]13C = -9.4 to -17.8[per mille sign]; [delta]D = -102 to -189[per mille sign]) which is believed to be characteristic for an abiogenic production of CH4 by CO2-reduction (Fischer-Tropsch reactions). Depletions in the deuterium content of three CH4 samples (to -377%) are probably caused by unknown subsurface rock alteration processes. Secondary hydrogen isotope exchange processes between methane, hydrogen and water are most likely responsible for calculated unrealistic methane formation temperatures. We show that excess helium, slightly enriched in 3He, is present in the hydrothermal fluids emerging the seafloor of Paleohori Bay. When the isotopic ratio of the excess component is calculated a 3He/4Heexcess of 3.6 · 10-6 is obtained: This indicates that the excess component consists of about one third of mantle helium and two thirds of radiogenic helium. We infer that the mantle-derived component has been strongly diluted by radiogenic helium during the ascent of the fluids to the surface.

Constraints on origin and evolution of Red Sea brines from helium and argon isotopes

Brines from three depressions along the axis of the Red Sea, the Atlantis II, the Discovery and the Kebrit Deep, were sampled and analyzed for helium and argon isotopes. We identified two principally different geochemical fingerprints that reflect the geological setting of the deeps. The Atlantis II and the Discovery brines originating from locations in the central Red Sea show 4 He concentrations up to 1.2U10 35 cm 3 STP g 31 and a 3 He/ 4 He ratio of 1.27U10 35 . The MORB-like 3 He/ 4 He ratio is typical of an active hydrothermal vent system and clearly indicates a mantle origin of the helium component within the brines. 40 Ar/ 36 Ar ratios are as high as 305 implying that mantle-derived 40 Ar excesses of up to 3% of the total argon concentration are present in the brines and transported along with the mantle helium signal. The mean ( 4 He/ 40 Ar)excess ratio of 2.1 is in agreement with the theoretical mantle production ratio. In the Kebrit Deep, located in the northern Red Sea, we found a helium excess of 5.7U10 37 cm 3 STP g 31 . The low 3 He/ 4 He ratio of 1U10 36 points to a predominantly radiogenic source of the helium excess with only a minor mantle contribution of approximately 9%. We propose a new scenario assuming that the Kebrit brine accumulates a diffusive helium flux that migrates from deeper sedimentary or crustal horizons. In contrast to the Atlantis II and Discovery Deep, the Kebrit brine shows no sign of an active hydrothermal input. fl 2001 Elsevier Science B.V. All rights reserved.

Equatorial Pacific productivity and dust flux during the mid‐Pleistocene climate transition

Received 6 May 2005; revised 30 August 2005; accepted 19 September 2005; published 17 December 2005. [1] We present a helium isotope record for core TT013-114PC from the central equatorial Pacific (140� W, 4� N, 4432 m water depth) spanning a period of 1 million years. We focus on the time interval from 560 to 800 kyr, largely coinciding with the mid-Pleistocene climate transition (MPT) when the dominant period of the Earth’s climate variability shifted from 41 kyr to 100 kyr. The terrigenous 4 He concentrations from our study correlate very well with published titanium concentrations in this core strongly supporting the use of terrigenous 4 He as a monitor of continental dust. Normalizing titanium and terrigenous 4 He concentrations to 3 He suggests that the dust supply during the MPT was approximately 30% lower compared to the subsequent period (560–100 kyr). The 3 He-normalized barium, aluminum and phosphorus concentrations, trace elements with a predominantly biogenic source in these sediments, are relatively constant. This is in contrast to previous studies that reported an apparent rise of titanium-normalized productivity proxies. Rather than a significant increase in productivity during the MPT, we conclude that the dust flux to the central equatorial Pacific was reduced and that the export productivity was approximately constant during this period of climate reorganization.

Origin of trace gases in submarine hydrothermal vents of the Kolbeinsey Ridge, north Iceland

Two hydrothermal fields of the Kolbeinsey Ridge area, north of Iceland, show vent gas characteristics which can be related to the subsurface conditions. Helium isotopes (R/R-air = 9.8, 10.9) indicate a mantle-derived origin and can be considered as a mixture of MORE helium and a deep-mantle plume helium component. The carbon isotope composition of CO2 ranges between -2.4 and -7.8 parts per thousand. The less negative delta(13)C-CO2 values were-found at Grimsey. The data from Grimsey are very similar to those previously published and regarded as being characteristic for the Icelandic magmatic source. However, small amounts of biogenic CO2 and/or subsurface calcite precipitation are responsible for the lighter isotope values of CO2 from Kolbeinsey. CH4/He-3 ratios which are higher than in MORB indicate an additional (sedimentary) methane source for Kolbeinsey and Grimsey hydrothermal gases. The presence of higher hydrocarbons up to butane, together with the carbon isotope values of methane (delta(13)C = -26.1 to -39.8 parts per thousand) suggest a probably high-mature organic source within thick sediments of the Tjornes Fracture Zone and smaller depressions on the west side of the Kolbeinsey Ridge crest. Geochemical characteristics of hydrocarbons present in KR hydrothermal fluids are, however, typical for a mixed (thermogenic and high-temperature hydrothermal, e.g. EPR-type) origin. Moreover, it is likely that secondary processes such as bacterial oxidation and thermal cracking determined the geochemical characteristics of the gases