Jordan F. Clark

Cooling of tropical Brazil (5°C) during the last glacial maximum

A 30,000-year paleotemperature record derived from noble gases dissolved in carbon-14-dated ground water indicates that the climate in lowland Brazil (Piaui Province, 7�S, 41.5�W; altitude, 400 meters) was 5.4� � 0.6�C cooler during the last glacial maximum than today. This result suggests a rather uniform cooling of the Americas between 40�S and 40�N. A 5.4�C cooling of tropical South America is consistent with pollen records, snow line reconstructions, and strontium/calcium ratios and δ18O coral records but is inconsistent with the sea-surface temperature reconstruction of CLIMAP (Climate: Long-Range Investigation, Mapping and Prediction). On the basis of these results, it appears that the tropical Americas are characterized by a temperature sensitivity comparable to that found in higher latitudes.

Paleotemperatures in the Southwestern United States Derived from Noble Gases in Ground Water

A paleotemperature record based on measurements of atmospheric noble gases dissolved in ground water of the Carrizo aquifer (Texas) shows that the annual mean temperature in the southwestern United States during the last glacial maximum was about 5�C lower than the present-day value. In combination with evidence for fluctuations in mountain snow lines, this cooling indicates that the glacial lapse rate was approximately the same as it is today. In contrast, measurements on deep-sea sediments indicate that surface temperatures in the ocean basins adjacent to our study area decreased by only about 2�C. This difference between continental and oceanic records poses questions concerning our current understanding of paleoclimate and climate-controlling processes.

Quantifying groundwater discharge to Cockburn River, southeastern Australia, using dissolved gas tracers 222Rn and SF6

Groundwater discharge to the Cockburn River, southeast Australia, has been estimated from comparison of natural 222Rn activities in groundwater and river water, interpreted using a numerical flow model that simulates longitudinal radon activities as a function of groundwater inflow, hyporheic exchange, evaporation, gas exchange with the atmosphere, and radioactive decay. An injection of SF6 into the river to estimate the gas transfer velocity assisted in constraining the model. Previous estimates of groundwater inflow using 222Rn activities have not considered possible input of radon due to exchange between river water and water in the hyporheic zone beneath the streambed. In this paper, radon input due to hyporheic exchange is estimated from measurements of radon production by hyporheic zone sediments and rates of water exchange between the river and the hyporheic zone. Total groundwater inflow to the Cockburn River is estimated to be 18500 m 3/d, although failure to consider hyporheic exchange would cause overestimation of the volume of groundwater inflow by approximately 70%. Copyright 2006 by the American Geophysical Union.

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

Low 14C activities in groundwater of the confined part of the Aquia aquifer in southeastern Maryland suggest that most of this water infiltrated at least 30,000 years ago. However, radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange, possibly with secondary calcite deposits in the formation, leading to overestimated 14C ages. Whereas the geochemistry of the Aquia aquifer complicates the application of the widely used 14C dating method, the accumulation of radiogenic He seems to provide a viable alternative for establishing a chronology. The quasi-linear increase of He concentrations with flow distance observed in the Aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic He. U and Th concentrations in Aquia sand were measured in order to determine the accumulation rate of 4He with sufficient confidence to establish a He time scale. Concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration. These noble gas temperatures (NGTs) clearly show the presence of water that infiltrated under much cooler conditions than at present. NGTs are correlated with chloride concentrations, corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal. In contrast, the stable isotope ratios δ180 and δD do not provide a clear record of past climatic changes in the Aquia aquifer and the correlation between NGTs and stable isotope ratios is weak. The NGT record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum (LGM) were (9.0 ± 0.6) °C colder than during the Holocene. This difference is slightly lower than estimates derived from pollen data for this region, but considerably larger than the rather uniform cooling of about 5°C indicated by noble gas studies in more southern locations of North America. The larger cooling is ascribed to the influence of the Laurentide ice sheet, which at its maximum extension came as close as 250 km to our study site.

Natural marine seepage blowout: Contribution to atmospheric methane

The release of methane sequestered within deep-sea methane hydrates is postulated as a mechanism for abrupt climate change; however, whether emitted seabed methane reaches the atmosphere is debatable. We observed methane emissions for a blowout from a shallow (22 m) hydrocarbon seep. The emission from the blowout was determined from atmospheric plume measurements. Simulations suggest a 1.1% gas loss to dissolution compared to ∼ 10% loss for a typical low-flux bubble plume. Transfer to the atmosphere primarily was enhanced by the rapid upwelling flows induced by the massive discharge. This mechanism could allow methane suddenly released from deeper (>250 m) waters to contribute significantly to atmospheric methane budgets. Copyright 2006 by the American Geophysical Union.

Temporal variation in natural methane seep rate due to tides, Coal Oil Point area, California

Two large steel tents (each 30 m by 30 m), open at the bottom to the seafloor, capture ∼16,800 m3 d-1 (594 MCF) of primarily methane from a large natural hydrocarbon seep, occurring a kilometer offshore in 67 m of water. The gas is piped to shore where it is metered and processed. The seep flow rate was monitored hourly for 9 months. Our results show that the tidal forcing causes the flow rate to vary by 4-7% around the mean. These results are the first quantitative documentation of the effect of tides on natural gas seepage in relatively deep water. Time series analyses of the 9 month record clearly show four principal tidal components with periods of 12.0, 12.4, 23.9, and 25.8 hours. High tide correlates with reduced flow, and low tide correlates with increased flow. The correlation indicates that each meter increase of sea height results in a decrease of 10-15 m3 hr-1 or 1.5-2.2% of the hourly flow rate. The observed changes are best accounted for by a pore activation model, whereby gas is released from small pores at low pressures but is inhibited at higher pressure. Pressure-dependent gas solubility changes are a less likely cause of flow variation. Our study implies that sea level differences, on a tidal timescale, can significantly change the gas seepage rate from sediments. Lower sea level in the last hundred thousand years would presumably allow higher gas loss from the sediment, assuming sufficient gas present, because of reduced hydrostatic pressure at the sediment-sea interface. The magnitude of this long-term change cannot be extrapolated from our tidal data. Copyright 2001 by the American Geophysical Union.

Dissolved hydrocarbon flux from natural marine seeps to the southern California Bight

Natural marine seepage near Coal Oil Point, Santa Barbara Channel, California, injects large quantities of hydrocarbons into the coastal ocean. The dispersal and source strength of the injected methane, ethane, and propane from this seep field was determined using a variety of oceanographic and geochemical techniques. The results show that hydrocarbons seep into stratified coastal waters creating plumes that extend for at least 12 km. The plume structure is complex because of the large geographical distribution of seep vents and because of the chaotic nature of advection and mixing near the seeps. At the time of the survey, hydrocarbons were injected onto density surfaces between σθ = 24.5–26.0 kg m−3. Earlier work has shown that subsurface methane maxima in the upper waters of the southern California Bight are typically found on these density surfaces. We estimate that the total flux of methane into the water column above the Coal Oil Point seeps is 2×1010 g yr−1 and is approximately equal to the total flux of dissolved methane to the atmosphere estimated for the entire southern California Bight. These observations strongly support the inference of others that coastal sources, which include some of the worlds largest marine hydrocarbon seeps, maintain the methane maximum observed offshore California. Estimates of the global methane flux from coastal waters derived by extrapolating the flux from coastal California may be too large because of the anomalous amount of marine hydrocarbon seepage in these waters.

Chemical evolution of shallow groundwater as recorded by springs, Sagehen basin; Nevada County, California

Abstract Springs in Sagehen basin, California, were used to document the effect of chemical weathering on the chemical evolution and composition of groundwater in a high elevation catchment. Geochemical tracer ages were determined with chlorofluorocarbons (CFCs) and tritium/ 3 He dating techniques. The spring water ages range from less than 5 years to almost 40 years. Mass balance calculations performed by NETPATH were combined with spring water ages to calculate chemical weathering rates observed throughout the basin, which range from 0.0116 to 0.0018 and from 0.0036 to 0.0006 mmol l −1 year −1 , for plagioclase and hornblende, respectively. Major cation concentrations, pH, and spring water conductivity were found to correlate positively ( R 2 =0.7) with spring water age. This suggests that shallow groundwater, as represented by the springs, is a chemically evolving system.

Decrease in natural marine hydrocarbon seepage near Coal Oil Point, California, associated with offshore oil production

Prolific natural hydrocarbon seepage occurs offshore of Coal Oil Point in the Santa Barbara Channel, California. Within the water column above submarine vents, plumes of hydrocarbon gas bubbles act as acoustic scattering targets. Using 3.5 kHz sonar data, seep distribution offshore of Coal Oil Point was mapped for August 1996, July 1995, and July 1973. Comparison of the seep distributions over time reveals more than 50% decrease in the areal extent of seepage, accompanied by declines in seep emission volume, in a 13 km 2 area above a producing oil reservoir. Declines in reservoir pressure and depletion of seep hydrocarbon sources associated with oil production are the mechanisms inferred to explain the declines in seep area and emission volume.

Dissolved methane distributions and air‐sea flux in the plume of a massive seep field, Coal Oil Point, California

in the down current surface water at 79 stations in a 280 km 2 study area. The methane plume spread over an area of � 70 km 2 and emitted on the order of 5 � 10 4 mol d � 1 to the atmosphere. A monthly time series at 14 stations showed variable methane concentrations which were correlated with changing sub-mesoscale surface currents. Air-sea fluxes estimated from the time series indicate that the air-sea flux derived for the 280 km 2 area is representative of the daily mean flux from this area. Only 1% of the dissolved methane originating from Coal Oil Point enters the atmosphere within the study area. Most of it appears to be transported below the surface and oxidized by microbial activity. Citation: Mau, S., D. L. Valentine, J. F. Clark, J. Reed, R. Camilli, and L. Washburn (2007), Dissolved methane distributions and air-sea flux in the plume of a massive seep field, Coal Oil Point, California, Geophys. Res. Lett., 34, L22603,