'Sediment'–cement relationships in a Pleistocene speleothem from Italy: a possible analogue for 'replacement' cements and Archaeolithoporella in ancient reefs

Ancient carbonate buildups may contain extraordinarily large amounts of early diagenetic precipitates. In some, host rock lamination may be traced into inclusion bands within the 'cement' crystals, suggesting that the crystals are replacive. By analogy with a Pleistocene speleothem from the Sorrento Peninsula, however, these relationships can be explained differently. In the speleothem, large, repeatedly split and dendritic calcite crystals occur within a laminated carbonate. Lamination consists of sub-mm alternations of micrite and microspar. Micritic laminae pass laterally into inclusion-rich growth bands in the dendritic calcite crystals, and have replaced an aragonitic cement, whereas the microspar laminae were primary calcite cements. Three types of inclusion-rich bands occur in the dendrite crystals: (1) with aragonite relicts, (2) 'ribbon calcite' and (3) with oriented micropores. When aragonite precipitated, the calcite dendrite branches were unable to keep growing as single crystals and split into crystallites (separated by micropores, some forming ribbon calcite), whereas during episodes of calcite lamina precipitation, the larger crystals were regenerated by crystallite coalescence. Calcite crystals are primary: they did not replace a micritic precursor. By analogy with the Italian speleothem, some ancient reefal sparry carbonates may not be replacements of earlier laminated sediments, but may have grown concurrently with them. It is also probable that some ancient laminated sediments were instead sea-floor precipitates, and that stromatolites containing cross-cutting crystal fabrics, and the alternating micrite-microspar laminae typical of Archaeolithoporella , could be largely abiotic crystal growths.     

Import of coastally-derived chlorophylla to South Slough, Oregon

Material transfer between estuaries and the nearshore zone has long been of interest, but information on the processes affecting Pacific Northwest estuaries has lagged behind other areas. The west coast of the U.S. is a region of seasonally variable upwelling that results in enhanced phytoplankton production in the nearshore zone. We examined estuarine-nearshore links over time by measuring physical oceanographic variables and chlorophylla concentration from an anchor station in South Slough, Oregon. Data was collected during 24-h cruises conducted at approximately weekly intervals during summer 1996 and spring 1997. The results demonstrate that the physical oceanography of this estuarine site was strongly influenced by the coastal ocean. Marine water reached the estuarine site on every sampled tide, and chlorophylla was clearly advected into the estuary with this ocean water. In contrast, phytoplankton concentrations were comparatively reduced in the estuarine water. There were, however, large fluctuations in the import of chlorophyll over the course of the summer. These variations likely reflect upwelling-generated phytoplankton production in the coastal ocean and subsequent cross-shelf transport to the estuary. Suspension feeding organisms in South Slough likely depend on the advection of this coastally-derived phytoplankton. The large allochthonous chlorophyll input measured for this system appears dissimilar from most estuaries studied to date. Previous investigations have focused on the outwelling and inwelling of materials in estuaries. We must now consider the influence of coastal upwelling and downwelling processes on estuarine material exchange.     

Aseismio fault slip and block deformation in North China

In North China, the tectonic fault-block system enables us to use the Discontinuous Deformation Analysis (DDA) method to simulate the long-term cross-fault survey and other geodetic data related to aseismic tectonic deformation. By the simulation we have found that: (1) Slips on faults with different orientation are generally in agreement with the ENE-WSW tectonic stress field, but the slip pattern of faulting can vary from nearly orthogonal, to pure shear along the strike of the faults, this pattern cannot be explained by simple geometric relation between the strike of the fault and the direction of the tectonic shortening. This phenomenon has been observed at many sites of cross-fault geodetic surveys, and might be caused by the interactions between different blocks and faults. (2) According to the DDA model, if the average aseismic slip rate along major active faults is at the order of several tenths of millimeter per year as observed by the cross-fault geodetic surveys, the typical strain rate inside a block is at the order of 10^–8 year^–1 or less, so that the rate of 10^–6 year^–1, as reported by observations in smaller areas, cannot be the representative deformation rate in this region. (3) Between the slips caused by regional compression and block rotation, there is a possibility that the sense of slip caused by rigid body rotation in two adjacent blocks is opposite to the slip caused by the tectonic compression. But the magnitude of slip resulting from the tectonic compression is much larger than that due to the block rotation. Thus, in general, the slip pattern on faults as a whole agrees with the sense of tectonic compression in this region. That is to say, the slip caused by regional compression dominates the entire slip budget. (4) Based on (3), some observed slips in contradiction to ENE tectonic stress field may be caused by more localized sources, and have no tectonic significance.     

Tropical soils could dominate the short-term carbon cycle feedbacks to increased global temperatures

Results of a simple model of the effects of temperature on net ecosystem production call into question the argument that the large stocks of soil carbon and greater projected warming in the boreal and tu ndra regions of the world willlead to rapid efflux of carbon from these biomes to the atmosphere. We show that low rates of carbon turnover in these regions and a relatively greater response of net primary production to changes in temperature may lead to carbon storage over some limited range of warming. In contrast, the high rates of soil respiration found in tropical ecosystems are highly sensitive to small changes in temperature, so that despite the less pronounced warming expected in equatorial regions, tropical soils are likely to release relatively large amounts of carbon to the atmosphere. Results for high-latitude biomes are highly sensitive to parameter values used, while the net efflux of carbon from the tropics appears robust.     

Groundwater quality variations in glacial drift and bedrock aquifers,Barry County,Michigan, U.S.A.

Groundwater samples from 288 domestic wells in Barry County, Michigan, were analyzed for 33 inorganic chemical parameters. Variations in chemical composition were investigated by considering the possible effects of human impact, aquifer type (bedrock vs glacial drift), chemical evolution along groundwater flow paths, and glacial landform type (moraine vs outwash). Approximately 25 percent of the glacial drift wells were classified as degraded by human impact and were excluded from further analysis of chemical variation. Two-sample tests comparing individual concentrations from drift and bedrock aquifers suggest that groundwater in the Marshall Sandstone aquifer is derived from local recharge through the glacial drift. This conclusion is supported by generalized groundwater flow patterns recognized for the two aquifers.Concentrations in both aquifers were examined in relation to generalized flow paths derived from water level data and also by classification of wells as recharge, transition, and discharge. No spatial concentration trends in major ions were detected, although iron concentrations do appear to increase from recharge to discharge areas. Declining redox potential along groundwater flow paths may explain this trend.The possible influence of glacial landform type was investigated by comparing concentrations of wells in moraines with those in outwash deposits. Wells in moraines have significantly higher concentrations of most parameters, perhaps due to higher content of finer, more chemically reactive sediment grains.     

Nd isotopic evidence for the position of southernmost Indian terranes within East Gondwana

Sm-Nd model ages of orthopyroxene-bearing massif charnockites from the Cardamom Hills Massif and adjoining supracrustal rocks from the Kerala Khondalite Belt in southernmost India are used to infer some of the relationships within these rocks and between them and neighboring areas. Most of these rocks have model ages of 2.1–2.8 Ga with most charnockites in the range 2.2–2.6 Ga. Thus, 3.0–3.4 Ga Archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks. The similarity of Sm-Nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks. A cordierite gneiss, representative of a relatively minor lithology in the supracrustal belt, has a model age of 1.3 Ga. The protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks. The source material of the gneiss protolith may have been located in the Wanni and Vijayan Complexes of Sri Lanka. The overlap of the model ages of rocks in this area and those in the Highland Complex of Sri Lanka supports the notion that these two sets of rocks were joined to each other in Gondwana. They belong to a belt that ran from Antarctica through Sri Lanka and India into Madagascar. This belt was involved in Pan-African tectono-metamorphism, as reflected in the 550 Ma age of the last, granulite-forming, event throughout the belt.