Geochemistry, Geophysics, Geosystems

Hetzinger, Steffen; Pfeiffer, Miriam; Dullo, Wolf‐Christian; Ruprecht, Eberhard; Garbe‐Schönberg, Dieter

doi: 10.1029/2006GC001347pmid: N/A

This study provides the first monthly resolved, 41‐year record of geochemical variations (δ18O and Sr/Ca) in a fast‐growing Diploria strigosa brain coral from Guadeloupe, Caribbean Sea. Linear regression yields a significant correlation of coral Sr/Ca (δ18O) with instrumental sea surface temperature (SST) on both monthly and mean annual scales (e.g., r = −0.59 for correlation between Simple Ocean Data Assimilation (SODA) SST and Sr/Ca, and r = −0.66 for δ18O; mean annual scale, p < 0.0001). The generated coral Sr/Ca (δ18O)‐SST calibration equations are consistent with each other and with published equations using other coral species from different regions. Moreover, a high correlation of coral Sr/Ca and δ18O with local air temperature on a mean annual scale (r = −0.78 for Sr/Ca; r = −0.73 for δ18O; p < 0.0001) demonstrates the applicability of geochemical proxies measured from Diploria strigosa corals as reliable recorders for interannual temperature variability. Both coral proxies are highly correlated with annual and seasonal mean time series of major SST indices in the northern tropical Atlantic (e.g., r = −0.71 for correlation between the index of North Tropical Atlantic SST anomaly and Sr/Ca, and r = −0.70 for δ18O; mean annual scale, p < 0.001). Furthermore, the coral proxies capture the impact of the El Niño–Southern Oscillation on the northern tropical Atlantic during boreal spring. Thus fast‐growing Diploria strigosa corals are a promising new archive for the Atlantic Ocean.

Rühlemann, Carsten; Butzin, Martin

doi: 10.1029/2006GC001251pmid: N/A

Alkenone temperatures derived from suspended particulate organic material which was collected in austral summer 2001 from surface waters (5 m) south of the Brazil‐Malvinas Confluence deviate from measured temperatures by −4° to −7°C when UK′37 ratios are converted to temperature using the Müller et al. (1998) calibration and up to −3°C when using the calibration of Conte et al. (2006). In contrast, alkenone temperatures determined from particulate material sampled north of the confluence reveal close correspondence to in situ temperatures or show slightly higher values. We suggest that the southern samples are biased by suspended organic detritus originating from the cold subpolar waters of the northward flowing Malvinas Current, whereas the northern samples carry an UK′37 signal of tropical/subtropical origin, transported southward with the Brazil Current. On the basis of surface ocean transport pathways and velocities simulated with the large‐scale geostrophic (LSG) ocean general circulation model, we identify areas of the world ocean where alkenone temperatures are potentially biased to higher or lower values due to long particle residence times and lateral advection by surface currents.

Tomaru, Hitoshi; Torres, Marta E.; Matsumoto, Ryo; Borowski, Walter S.

doi: 10.1029/2005GC001207pmid: N/A

Because gas hydrate is preferentially enriched in the heavy water isotopes, the δ18O and δD values of pore waters collected from gas hydrate–bearing sediment can provide information on the abundance and mechanisms of gas hydrate formation. Pore waters sampled from deep‐seated (40 to 125 mbsf) gas hydrate deposits in Hydrate Ridge during ODP Leg 204 show depletion in dissolved Cl− and enrichments in 18O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (αO = 1.0025 and αH = 1.022) estimated from an extensive data set (n = 30 samples) correspond to experimentally determined values. In contrast, pore waters from shallow samples (<25 mbsf) at the ridge summit (n = 32) are highly enriched in dissolved Cl− and depleted in 18O and D, consistent with formation of massive gas hydrate deposits at rates faster than those at which these anomalies would be removed by advection or diffusion. The water isotopic fractionation factors in the brine are significantly lower than those experimentally determined, with αO of 1.0010 (average value of 1.0012) and αH of 1.008 (average value of 1.008). We discuss several factors that may be causing this anomalous fractionation and suggest that low gas occupancy in hydrate lattice (high hydration number) may be responsible for the observed small fractionation. If this were the case, the oxygen and hydrogen fractionation may serve as an indicator of hydration number during formation of gas hydrate in natural systems.

Clift, Peter D.; Carter, Andrew; Campbell, Ian H.; Pringle, Malcolm S.; Van Lap, Nguyen; Allen, Charlotte M.; Hodges, Kip V.; Tan, Mai Thanh

doi: 10.1029/2006GC001336pmid: N/A

Sand samples from the mouths of the Red and Mekong Rivers were analyzed to determine the provenance and exhumation history of their source regions. U‐Pb dating of detrital zircon grains shows that the main sources comprise crust formed within the Yangtze Craton and during the Triassic Indosinian Orogeny. Indosinian grains in the Mekong are younger (210–240 Ma) than those in the Red River (230–290 Ma), suggesting preferential erosion of the Qiangtang Block of Tibet into the Mekong. The Red River has a higher proportion of 700–800 Ma grains originally derived from the Yangtze Craton. 40Ar/39Ar dating of muscovite grains demonstrates that rocks cooled during the Indosinian Orogeny are dominant in both rivers, although the Mekong also shows a grain population cooling at 150–200 Ma that is not seen in the Red River and which is probably of original Qiangtang Block origin. Conversely, the Red River contains a significant mica population (350–500 Ma) eroded from the Yangtze Craton. High‐grade metamorphic rocks exposed in the Cenozoic shear zones of southeast Tibet‐Yunnan are minority sources to the rivers. However, apatite and zircon fission track ages show evidence for the dominant sources, especially in the Red River, only being exhumed through the shallowest 5–3 km of the crust since ∼25 Ma. The thermochronology data are consistent with erosion of recycled sediment from the inverted Simao and Chuxiong Basins, from gorges that incise the eastern flank of the plateau. Average Neogene exhumation rates are 104–191 m/Myr in the Red River basin, which is within error of the 178 ± 35 m/Myr estimated from Pleistocene sediment volumes. Sparse fission track data from the Mekong River support the Ar‐Ar and U‐Pb ages in favoring tectonically driven rock uplift and gorge incision as the dominant control on erosion, with precipitation being an important secondary influence.

Rhie, Junkee; Romanowicz, Barbara

doi: 10.1029/2006GC001274pmid: N/A

We previously showed that the Earth's “hum” is generated primarily in the northern oceans during the northern hemisphere winter and in the southern oceans during the summer. To gain further insight into the process that converts ocean storm energy into elastic energy through coupling of ocean waves with the seafloor, we here investigate a 4‐day‐long time window in the year 2000 that is free of large earthquakes but contains two large “hum” events. From a comparison of the time functions of two events and their relative arrival times at the two arrays in California and Japan, we infer that the generation of the “hum” events occurs close to shore and comprises three elements: (1) short‐period ocean waves interact nonlinearly to produce infragravity waves as the storm reaches the coast of North America; (2) infragravity waves interact with the seafloor locally to generate long‐period Rayleigh waves; and (3) some free infragravity wave energy radiates out into the open ocean, propagates across the north Pacific basin, and couples to the seafloor when it reaches distant coasts northeast of Japan. We also compare the yearly fluctuations in the amplitudes observed on the two arrays in the low‐frequency “hum” band (specifically at 240 s) and in the microseismic band (2–25 s). During the winter, strong correlation between the amplitude fluctuations in the “hum” and microseismic bands at BDSN is consistent with a common generation mechanism of both types of seismic noise from nonlinear interaction of ocean waves near the west coast of North America.

Sleep, Norman H.

doi: 10.1029/2006GC001374pmid: N/A

Frictional sliding dilates gouge or increases the separation of sliding surfaces. A common hypothesis is that the rate of dilatational strain scales linearly with the rate of shear strain. The proportionality constant is called the dilatancy coefficient. Real contact theory of friction may explain this feature. Moving contact asperities produce damaged regions with elastic strains scaling to the ratio of the real strength of asperities to the elastic modulus. Balance between this local strain‐energy production rate and macroscopic work against normal traction indicates that the dilatancy coefficient scales with this ratio. So do the average slopes on a mature rough sliding surface if opening‐mode cracks are unimportant. The result is compatible with the observed dilatancy coefficient of quartz gouge, ∼4%.

Andreasen, Dyke H.; Sosdian, Sindia; Perron‐Cashman, Suzanne; Lear, Caroline H.; deGaridel‐Thoron, Thibault; Field, Paul; Rosenthal, Yair

doi: 10.1029/2005GC001124pmid: N/A

Improving interlaboratory reproducibility (in both precision and accuracy) of Mg/Ca and Sr/Ca determination in marine biogenic carbonates is critical in optimizing their utility as paleothermometers. Coupled with a need for uniform sample cleaning practices, there is a need for more exacting methods and procedures across laboratories using varied instrumentation. Here we employ an intensity ratio/matrix‐effect correction methodology to a suite of solution standards and biogenic carbonates (foraminifera tests and a gastropod shell) to investigate short‐term and long‐term Mg/Ca and Sr/Ca precision and accuracy by different instruments: a magnetic‐sector inductively coupled plasma–mass spectrometer (ICP‐MS) and a radially viewed inductively coupled plasma–optical emission spectrophotometer (ICP‐OES). Over an extended 1.0–24.5 mM Ca concentration range, both instruments have significant Ca matrix effects for Mg/Ca and somewhat less for Sr/Ca. Over our working Ca range (1–8 mM Ca), Mg/Ca matrix effects are significant, requiring correction, and Sr/Ca matrix effects are small to negligible, occasionally requiring correction (linear or logarithmic fit) using a suite of matrix standards for both instruments. The short‐term (intrarun) precision for a suite of solution standards is <0.2% (1σ %RSD) for Mg/Ca and Sr/Ca for both instruments. A long‐term (interrun) precision of <0.9% is demonstrated for Mg/Ca and <0.6% for Sr/Ca on both instruments. The accuracy of measured Mg/Ca and Sr/Ca values for short‐ and long‐term standards is similar on both instruments at better than 1 ± 0.5%, on par with our long‐term precision. An interinstrument comparison of the same measured suite of biogenic carbonates demonstrates that after accounting for matrix effects, data generated on either instrument are essentially interchangeable (within analytical precision) to a high degree of fidelity.

Liu, Lianwen; Chen, Jun; Ji, Junfeng; Chen, Yang; Balsam, William

doi: 10.1029/2005GC001188pmid: N/A

Most of our knowledge concerning the evolution of the East Asian monsoon (EAM) comes from the Chinese loess‐paleosol sequence. Variations in the EAM, as recorded in loess, are highly correlated to the marine δ18O record and show a strong 100 kyr cyclicity during the past 800 kyr. This conclusion is based on both weathering and pedogenesis intensity changes as well as grain size variation in loess‐paleosol sequences which usually serves as a proxy for winter monsoon variation. However, grain size distributions are subject to modification by pedogenic processes. In this paper we used the Zr/Rb ratio from the Lingtai section on the Loess Plateau in central China to trace variations in the East Asian winter monsoon for the past 1.8 Myr. Long‐term Zr/Rb variations in the Lingtai section exhibit a similar amplitude and frequency to those of mean grain size in bulk samples over the past 1.8 Myr. Comparing the Zr/Rb record and mean grain size record between L1 and L9 shows that a good linear relationship exists in the glacial age loess samples, whereas paleosol samples usually deviate from the linear relationship because of intense pedogenesis during the relatively warm and humid interglacial intervals. Our results indicate that the Zr/Rb ratio reflects the original eolian grain size and may serve as a proxy for the strength of the East Asian winter monsoon. In the Lingtai section over the past 1.8 Myr, the Zr/Rb ratio record displays a very strong 41 kyr obliquity periodicity from 1.8–1.2 Myr B.P., implying that variations of the winter monsoon during this interval are controlled by high‐latitude climate. However, during the past 0.8 Myr the Zr/Rb ratio exhibits a stronger precession cycle influence, suggesting a possible low‐latitude control on the East Asian monsoon regime during this interval.

Lawrence, Jesse F.; Wiens, Douglas A.; Nyblade, Andrew A.; Anandakrishnan, Sridhar; Shore, Patrick J.; Voigt, Donald

doi: 10.1029/2006GC001282pmid: N/A

This study uses seismic receiver functions, surface wave phase velocities, and airborne gravity measurements to investigate the structure of the Transantarctic Mountains (TAM) and adjacent regions of the Ross Sea (RS) and East Antarctica (EA). Forty‐one broadband seismometers deployed during the Transantarctic Mountain Seismic Experiment provide new insight into the differences between the TAM, RS, and EA crust and mantle. Combined receiver function and phase velocity inversion with niching genetic algorithms produces accurate crustal and upper mantle seismic velocity models. The crustal thickness increases from 20 ± 2 km in the RS to a maximum of 40 ± 2 km beneath the crest of the TAM at 110 ± 10 km inland. Farther inland, the crust of EA is uniformly 35 ± 3 km thick over a lateral distance greater than 1300 km. Upper mantle shear wave velocities vary from 4.5 km s−1 beneath EA to 4.2 km s−1 beneath RS, with a transition between the two at 100 ± 50 km inland near the crest of the TAM. The ∼5 km thick crustal root beneath the TAM has an insufficient buoyant load to explain the entire TAM uplift, suggesting some portion of the uplift may result from flexure associated with a buoyant thermal load in the mantle beneath the edge of the TAM lithosphere.

Buerki, P. R.; Jackson, Brian C.; Schilling, Tim; Rufer, Terry; Severinghaus, Jeffrey P.

doi: 10.1029/2006GC001341pmid: N/A

In order to eliminate the use of liquid helium for the extraction of atmospheric gases from polar ice cores, two units of a redesigned top load helium exchange gas cryostat were built and tested. The cryostats feature the shortest and largest diameter sample wells built to date, a base temperature below 7 Kelvin, and a sample well without baffles. The cryostats allowed shortening the length and thus increasing the gas pressure inside our sample tubes by 58% and increasing the amount of sample ending up in the mass spectrometer by 4.4%. The cryostats can either be used as mobile stand‐alone units for manual gas processing lines or integrated into a fully automated vacuum extraction and gas analysis line. For the latter application the cryostat was equipped with a custom‐designed automated changeover system.