954 resultados para Shallow-water carbonates
Resumo:
Reworked shallow-water foraminifers that settled on the upper slope of the central Great Barrier Reef at Site 821 (water depth, 212.6 m) were used as indicators of the paleoclimatic and paleoenvironmental conditions that have controlled the Pleistocene evolution of the adjacent platform. Throughout the 400-m-thick sequence drilled, the nature, composition, and distribution of the shallow-water foraminiferal assemblages studied indicate that (1) all the species recorded are at present living in diverse tropical, reef-related areas of the Indo-Pacific and Atlantic provinces; (2) the composition of the microfaunal taphocoenoses is almost identical between the different stratigraphic intervals studied and the modern Great Barrier Reef environments; (3) inner-neritic, tropical environments have continued to develop since the middle Pleistocene; (4) high- to moderate-energy platform edges occurred repeatedly throughout Pleistocene time. These factors may suggest that, since the beginning of the Pleistocene, several reef-like tracts have grown successively on the central area of the northeastern Australian shelf edge. These tracts probably had a sufficiently evolved morphological zonation to act as shelters for foraminiferal biocoenoses of high species diversity.
Resumo:
The West Antarctic Peninsula is one of the fastest warming regions on the planet. Faster glacier retreat and related calving events lead to more frequent iceberg scouring, fresh water input and higher sediment loads which may affect benthic marine communities. On the other hand, the appearance of newly formed ice-free areas provides new substrates for colonization. Here we investigated the effect of these conditions on four benthic size classes (microbenthos, meiofauna and macrofauna) using Potter Cove (King George Island, West Antarctic Peninsula) as a case study. We identified three sites within the cove experiencing different levels of glacier retreat-related disturbance. Our results showed the existence of different communities at the same depth over a relatively small distance (about 1 km**2). This suggests glacial activity structures biotic communities over a relatively small spatial scale. In areas with frequent ice scouring and higher sediment accumulation rates, a patchy community, mainly dominated by macrobenthic scavengers (such as Barrukia cristata), vagile organisms, and younger individuals of sessile species (such as Yoldia eigthsi) was found. Meiofauna organisms such as cumaceans are found to be resistant to re-suspension and high sedimentation loads. The nematode genus Microlaimus was found to be successful in the newly exposed ice-free site, confirming its ability as a pioneering colonizer. In general, the different biological size classes appear to respond in different ways to the ongoing disturbances, suggesting that adaptation processes may be size related. Our results suggest that with continued deglaciation, more diverse but less patchy macrobenthic assemblages can become established due to less frequent ice scouring events.
Resumo:
Ocean acidification causes biodiversity loss, alters ecosystems, and may impact food security, as shells of small organisms dissolve easily in corrosive waters. There is a suggestion that photosynthetic organisms could mitigate ocean acidification on a local scale, through seagrass protection or seaweed cultivation, as net ecosystem organic production raises the saturation state of calcium carbonate making seawater less corrosive. Here, we used a natural gradient in calcium carbonate saturation, caused by shallow-water CO2 seeps in the Mediterranean Sea, to assess whether seaweed that is resistant to acidification (Padina pavonica) could prevent adverse effects of acidification on epiphytic foraminifera. We found a reduction in the number of species of foraminifera as calcium carbonate saturation state fell and that the assemblage shifted from one dominated by calcareous species at reference sites (pH 8.19) to one dominated by agglutinated foraminifera at elevated levels of CO2 (pH 7.71). It is expected that ocean acidification will result in changes in foraminiferal assemblage composition and agglutinated forms may become more prevalent. Although Padina did not prevent adverse effects of ocean acidification, high biomass stands of seagrass or seaweed farms might be more successful in protecting epiphytic foraminifera.