Oceanographic controls on the carbon isotopic compositions of sinking particles from the Cariaco Basin

This study examined the relationship between carbon isotopic composition of sinking organic matter (OM) and the biological, physical and chemical properties of the surface ocean in the Cariaco Basin. The ¹³C/¹²C ratio of OM (δ¹³C_org) in sinking particles was determined on sediment trap samples from four depths collected from 1996 to 1999 as part of the CArbon Retention In A Colored Ocean time series. Water column properties, including temperature, productivity, chlorophyll and concentration of dissolved CO₂, were concurrently measured on monthly cruises. The δ¹³C_org varied from a high of –17.7‰ to a low of –22.6‰ during the study period. The variation of the δ¹³C_org throughout seasonal cycles was directly proportional to the strength of upwelling and was negatively correlated with temperature (r²=0.64). During the 1996–1997 upwelling event, the strongest during the study period, the δ¹³C_org increased by 4.4‰ whereas during the 1998–1999 upwelling event, the weakest during the study period, the δ¹³C_org only increased by 3.3‰. Contrary to most previous studies, we observed a negative relationship (r²=0.53) between [CO_2 aq] and the estimated isotopic fractionation factor (ε_p). However, there was no correlation between ε_p and the calculated growth rates indicating that there was non-diffusive uptake of carbon into phytoplankton cells. It thus appears that [CO_2 aq] does not control the δ¹³C_org in the water column of the study site. The best explanation for the isotopic enrichment observed is a carbon concentrating mechanism (CCM) in phytoplankton. The existence of a CCM in phytoplankton has major implications for the interpretation of the δ¹³C_org in the Cariaco Basin.     

Small volume andesite magmas and melt–mush interactions at Ruapehu, New Zealand: evidence from melt inclusions

Historical eruptions from Mt. Ruapehu (New Zealand) have been small (<0.001 km³ of juvenile magma) and have often occurred without significant warning. Developing better modelling tools requires an improved understanding of the magma storage and transport system beneath the volcano. Towards that end, we have analysed the volatile content and major element chemistry of groundmass glass and phenocryst-hosted melt inclusions in erupted samples from 1945 to 1996. We find that during this time period, magma has been stored at depths of ~2–9 km, consistent with inferences from geophysical data. Our data also show that Ruapehu magmas are relatively H₂O-poor (<2 wt%) and CO₂-rich (≤1,000 ppm) compared to typical arc andesites. Surprisingly, we find that melt inclusions are often more evolved than their transporting melt (as inferred from groundmass glass compositions). Furthermore, even eruptions that are separated by less than 2 years exhibit distinct major element chemistry, which suggests that each eruption involved magma with a unique ascent history. From these data, we infer that individual melt batches rise through, and interact with, crystal mush zones formed by antecedent magmas. From this perspective, we envision the magmatic system at Ruapehu as frequently recharged by small magma inputs that, in turn, cool and crystallise to varying degrees. Melts that are able to erupt through this network of crystal mush entrain (to a greater or lesser extent) exotic crystals. In the extreme case (such as the 1996 eruption), the resulting scoria contain melt inclusion-bearing crystals that are exotic to the transporting magma. Finally, we suggest that complex interactions between recharge and antecedent magmas are probably common, but that the small volumes and short time scales of recharge at Ruapehu provide a unique window into these processes.     

Geoarchaeological research in the coastal plain portion of the Savannah River valley

Selected archaeological, stratigraphical, sedimentological, hydrological, sea level, and ¹⁴C data are considered in order to ascertain the times and rates of saltmarsh, floodplain, river swamp, and alluvial terrace development in the lower Savannah River valley. Archaeological data are emphasized in the elucidation of these developmental trends and their environmental correlates. It is concluded that the modern environments associated with the categories of archaeological sites examined (estuarine shell middens, river swamp point bar sites, raised terrace/point bar sites) are not representative of local conditions during occupation(s). Accordingly, a mid-Holocene development of lower energy depositional environments is documented, along with a corresponding shift in subsistence-settlement patterning that suggests more diffuse, or broad spectrum, socio-economic strategies. A base level (sea level) dominance on river sedimentation, even far inland from the coast, may also be indicated.