In this study, we test various parameters in deep-sea sediments (bulk sediment parameters and changes in microfossil abundances and preservation character) which are generally accepted as indicators of calcium carbonate dissolution. We investigate sediment material from station GeoB 1710-3 in the northern Cape Basin (eastern South Atlantic), 280 km away from the Namibian coast, well outside today’s coastal upwelling. As northern Benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr (Volbers et al., submitted), GeoB 1710-3 sediments reflect these changes in upwelling productivity. Results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity. Accordingly, these conventional proxy parameters misrepresent, to some extent, the extent of calcium carbonate dissolution. These results were verified by an independent dissolution proxy, the Globigerina bulloides dissolution index (BDX′) (Volbers and Henrich, submitted). The BDX′ is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr, with the exception of one pronounced dissolution event at early oxygen isotopic stage (OIS) 6.
The early OIS 6 is characterized by calcium carbonate contents, sand contents, and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr. At the same time, the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased, as are the rain ratio, the fragmentation index, and the BDX′. The sedimentary calcite lysocline rose above the core position and GeoB 1710-3 sediments were heavily altered, as attested to by the unusual accumulation of pellets, aggregates, sponge spicules, radiolaria, benthic foraminifera, and planktonic foraminiferal assemblages.
Solely the early OIS 6 dissolution event altered the coarse fraction intensely, and is therefore reflected by all conventional calcium carbonate preservation proxies and the BDX′. We attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes: (a) a prominent change in the deep-water mass distribution within the South Atlantic and (b) intense degradation of organic material within the sediment (preserved as maximum total organic carbon content) creating microenvironments favorable for calcium carbonate dissolution. 相似文献
A high-resolution study of a marsh sedimentary sequence from the Minho estuary provides a new palaeoenvironmental reconstruction from NW Iberian based on geological proxies supported by historical and instrumental climatic records. A low-salinity tidal flat, dominated by Trochamminita salsa, Haplophragmoides spp. and Cribrostomoides spp., prevailed from AD 140–1360 (Roman Warm Period, Dark Ages, Medieval Climatic Anomaly). This sheltered environment was affected by high hydrodynamic episodes, marked by the increase in silt/clay ratio, decrease of organic matter, and poor and weakly preserved foraminiferal assemblages, suggesting enhanced river runoff. The establishment of low marsh began at AD 1380. This low-salinity environment, marked by colder and wet conditions, persisted from AD 1410–1770 (Little Ice Age), when foraminiferal density increased significantly. Haplophragmoides manilaensis and Trochamminita salsa mark the transition from low to high marsh at AD 1730. Since AD 1780 the abundances of salt marsh species (Jadammina macrescens, Trochammina inflata) increased, accompanied by a decrease in foraminiferal density, reflecting climate instability, when droughts alternate with severe floods. SW Europe marsh foraminifera respond to the hydrological balance, controlled by climatic variability modes (e.g., NAO) and solar activity, thus contributing to the understanding of NE Atlantic climate dynamics. 相似文献
I~IOXWith regard tO the 5,tudy on the planktonic copeal distributions in the Kuroshio waters ofthe East China Sea and its adjacent areas, a great ~nt of reSearch work has been made by somescholars at home (He and Yang, 1990; Meng et al., 1990; He, 1990; Liu et al., 1991; He etal., 1992; He et al., 1994). HOwever, the most of them were made in a single season, and it isabet about in the research of the various aspects of species comPOSition, abundance distributionand ecological characte… 相似文献
Rose Bengal stained benthic foraminifera were studied from 11 cores collected along two depth transects off southern Portugal: one in the Lisbon-Setúbal Canyon and the other along the canyon edge. The total standing stocks and distribution of foraminifera were investigated in relation to sediment and pore water geochemistry. Nitrate was used as a redox indicator, sedimentary chlorophyll a and CPE (chloroplastic pigment equivalents) contents as a measure of labile organic matter, and total organic carbon as a measure of bulk organic matter availability.The canyon sediments were enriched in organic carbon and phytopigments at all water depths in comparison with the canyon edge. Water depth seemed to control sedimentary phytopigment content, but not total organic carbon. No significant correlation was seen between pigment and total organic carbon content.The abundance of calcareous foraminifera correlated with the phytodetritus content, whereas a weaker correlation was observed for the agglutinated taxa. Therefore, calcareous foraminifera appear to require a fresher food input than agglutinated taxa. The foraminiferal species composition also varied with pigment content and nitrate penetration depth in the sediment, in line with the TROX concept. Phytopigment-rich (surficial CPE content >20 μg/cm3) sediments with a shallow nitrate penetration depth (∼1 cm depth) were inhabited by generally infaunal species such as Chilostomella oolina, Melonis barleeanus and Globobulimina spp. As the nitrate penetration increased to ∼2 cm depth in sediment and the pigment content remained relatively high (>15 μg/cm3), Uvigerina mediterranea and Uvigerina elongatastriata became dominant species. With declining CPE content and increasing nitrate penetration depth, the foraminiferal assemblages changed from the mesotrophic Cibicides kullenbergi-Uvigerina peregrina assemblage to the oligotrophic abyssal assemblage, mainly consisting of agglutinated taxa. 相似文献