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The occurrence and propagation of abrupt climate change between the high and low-latitudes has become an important focus of paleoclimatic and paleoceanographic research. The causes of abrupt change have significant implications for understanding future manifestations of similar forcings under late Holocene (‘Anthropocene’) boundary conditions. Of particular interest are signals indicative of sub-millennial scale climate change in the sub-tropics of similar magnitude and frequency to those recorded in Greenland ice cores. Earlier research in the Arabian Sea has highlighted the sensitivity of sedimentary organic carbon and nitrogen isotope measurements for recording the state of the SW monsoon and associated Arabian Sea Oxygen Minimum Zone. In this study, we exploit the unprecedented fidelity of the sedimentary δ15N record to identify a 20 cm interval at ODP Site 723 containing a stadial/inter-stadial interval between 43-42 Kyr BP. We employ sedimentary nitrogen isotopes, chlorin pigment and alkenone abundances, major and minor element analyses of highly-resolved (2 mm ≈ 10 yr) samples across this interval to compare a comprehensive, multi-proxy data set to understand (a) the processes contributing to the δ15N signal in the longer records of denitrification; and (b) the associated climatic events, especially the relative intensity of summer and winter monsoons at these times. A lack of evidence for bioturbation in excess of our 2 mm sampling resolution facilitates decadal-scale oceanographic and climatic reconstructions. Using a four-component flux-dilution model, we show that the deposition of carbonate decreased in parallel with an increase in Total Organic Matter flux from stadial to inter-stadial time. This interval is also marked by a significant drop in lithogenic (dust) accumulation, analogous to a similar decrease noted during deglaciation in the Western Arabian Sea. Combined with alkenone U37K′-derived estimates for sea surface temperature (SST), we conclude that the climatological shift from stadial to inter-stadial conditions at low latitudes was characterized by repeated switches in mean monsoon state approximately every 200 yr. The winter monsoon was the dominant mode during maximum stadial conditions; conversely the summer monsoon was dominant during maximum interstadial-like conditions. However, each interval was separated by a distinct ‘inter-monsoon’ mode, indicated by a higher continental dust flux but warmer SST. Proxy records for changing bottom-water oxygenation show near-identical results down to the mm-scale, but hint at increased export production leading the onset of anoxia during the stadial/inter-stadial transition. The coherence of all sedimentary signals depicts a wholesale reorganization of the Arabian Sea climate and marine ecosystem over approximately 200 years, a period that may be associated with monsoon modulation by small oscillations in solar irradiance.  相似文献   
2.
The Antarctic Ice Sheet plays a major role in the global system, and the large ice streams discharging into the circumpolar sea represent its gateways to the world’s oceans. Satellite radar altimeter data provide an opportunity for mapping surface elevation at kilometerresolution with meteraccuracy. Geostaristical methods have been developed for the analysis of these data. Applications to Seasat data and data from the Geosat Exact Repeat Mission indicate that the grounding line of Lambert Glacier/Amery Ice Shelf, the largest ice stream in East Antarctica, has advanced 10–12 km between 1978 and 1987–89. The objectives of this paper are to explore possibilities and limitations of satellite-altimetry-based mapping to capture changes for shorter time windows and for smaller areas, and to investigate some methodological aspects of the data analysis. We establish that one season of radar altimeter data is sufficient for constructing a map. This allows study of interannual variation and is the key for a limeseries analysis approach to study changes in ice streams. Maps of the lower Lambert Glacier and the entire Amery Ice Shelf are presented for austral winters 1978, 1987, 1988, and 1989. As a first step toward understanding the dynamics of the ice-stream/iceshelf system, elevation changes are calculated for grounded ice, the grounding zone, and floating ice. In the absence of (sufficient) surface gravity control for the Lambert Glacier/Amery Ice Shelf area, altimetry-based maps may facilitate improvement of geoid models as they provide constraints on the terrain correction in the inverse gravimetric problem.  相似文献   
3.
The Antarctic Ice Sheet plays a major role in the global system, and the large ice streams discharging into the circumpolar sea represent its gateways to the world's oceans. Satellite radar altimeter data provide an opportunity for mapping surface elevation at kilometer-resolution with meter-accuracy. Geostatistical methods have been developed for the analysis of these data. Applications to Seasat data and data from the Geosat Exact Repeat Mission indicate that the grounding line of Lambert Glacier/Amery Ice Shelf, the largest ice stream in East Antarctica, has advanced 10–12 km between 1978 and 1987–89. The objectives of this paper are to explore possibilities and limitations of satellite-altimetry-based mapping to capture changes for shorter time windows and for smaller areas, and to investigate some methodological aspects of the data analysis. We establish that one season of radar altimeter data is sufficient for constructing a map. This allows study of interannual variation and is the key for a time-series analysis approach to study changes in ice streams. Maps of the lower Lambert Glacier and the entire Amery Ice Shelf are presented for austral winters 1978, 1987, 1988, and 1989. As a first step toward understanding the dynamics of the ice-stream/ice-shelf system, elevation changes are calculated for grounded ice, the grounding zone, and floating ice. In the absence of (sufficient) surface gravity control for the Lambert Glacier/Amery Ice Shelf area, altimetry-based maps may facilitate improvement of geoid models as they provide constraints on the terrain correction in the inverse gravimetric problem.  相似文献   
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Ocean Dynamics - In the context of Canada’s Ocean Protection Plan (OPP), improved coastal and near-shore modelling is needed to enhance marine safety and emergency response capacity in the...  相似文献   
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A prerequisite for minimizing contamination risk whilst conducting managed aquifer recharge (MAR) with recycled water is estimating the residence time in the zone where pathogen inactivation and biodegradation processes occur. MAR in Western Australia’s coastal aquifers is a potential major water source. As MAR with recycled water becomes increasingly considered in this region, better knowledge of applied and incidental tracer-based options from case studies is needed. Tracer data were collected at a MAR site in Floreat, Western Australia, under a controlled pumping regime over a distance of 50 m. Travel times for bromide-spiked groundwater were compared with two incidental tracers in recycled water: chloride and water temperature. The average travel time using bromide was 87?±?6 days, whereas the estimates were longer based on water temperature (102?±?17 days) and chloride (98?±?60 days). The estimate of average flow velocity based on water temperature data was identical to the estimate based on bromide within a 25-m section of the aquifer (0.57?±?0.04 m day?1). This case study offers insights into the advantages, challenges and limitations of using incidental tracers in recycled water as a supplement to a controlled tracer test for estimating aquifer residence times.  相似文献   
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