941 resultados para Subtropical Estuary
Resumo:
Pollen analyses have been proven to possess the possibility to decipher rapid vegetational and climate shifts in Neogene sedimentary records. Herein, a c. 21-kyr-long transgression-regression cycle from the Lower Austrian locality Stetten is analysed in detail to evaluate climatic benchmarks for the early phase of the Middle Miocene Climate Optimum and to estimate the pace of environmental change. Based on the Coexistence Approach, a very clear signal of seasonality can be reconstructed. A warm and wet summer season with c. 204-236 mm precipitation during the wettest month was opposed by a rather dry winter season with precipitation of c. 9-24 mm during the driest month. The mean annual temperature ranged between 15.7 and 20.8 °C, with about 9.6-13.3 °C during the cold season and 24.7-27.9 °C during the warmest month. In contrast, today's climate of this area, with an annual temperature of 9.8 °C and 660 mm rainfall, is characterized by the winter season (mean temperature: -1.4 °C, mean precipitation: 39 mm) and a summer mean temperature of 19.9 °C (mean precipitation: 84 mm). Different modes of environmental shifts shaped the composition of the vegetation. Within few millennia, marshes and salt marshes with abundant Cyperaceae rapidly graded into Taxodiaceae swamps. This quick but gradual process was interrupted by swift marine ingressions which took place on a decadal to centennial scale. The transgression is accompanied by blooms of dinoflagellates and of the green alga Prasinophyta and an increase in Abies and Picea. Afterwards, the retreat of the sea and the progradation of estuarine and wetland settings were a gradual progress again. Despite a clear sedimentological cyclicity, which is related to the 21-kyr precessional forcing, the climate data show little variation. This missing pattern might be due to the buffering of the precessional-related climate signal by the subtropical vegetation. Another explanation could be the method-inherent broad range of climate-parameter estimates that could cover small scale climatic changes.
Resumo:
Benthic foraminiferal Cd/Ca from an intermediate depth, western South Atlantic core documents the history of southward penetration of North Atlantic Intermediate Water (NAIW). Cd seawater estimates (CdW) for the last glacial are consistent with the production of NAIW and its export into the South Atlantic. At ~14.5 ka concurrently with the onset of the Bølling-Allerød to Younger Dryas cooling, the NAIW contribution to the South Atlantic began to decrease, marking the transition from a glacial circulation pattern to a Younger Dryas circulation. High CdW in both the deep North Atlantic and the intermediate South Atlantic imply reduced export of deep and intermediate water during the Younger Dryas and a significant decrease in northward oceanic heat transport. A modern circulation was achieved at ~9 ka, concurrently with the establishment of Holocene warmth in the North Atlantic region, further supporting a close linkage between deepwater variability and North Atlantic climate.
