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River deltas occupy only ~0.65% of Earth's land surface, but collectively house ~4.5% of the global population and account for more than 6% of the global GDP. Because of ongoing human interventions in the past century (river diversions, groundwater and petroleum extraction, and urbanization), deltas are coming under additional and intense threat from climate change and the impending sea-level rise. Many highlatitude and tropical deltas where population pressure is low and human modification is minimal face less peril for the foreseeable future, but densely populated deltas, especially those in Asia with extreme urbanization and environmental pressures, will be more susceptible to land loss and drowning. Here we consider six key deltas--Mississippi, Yangtze, Niger, Bengal, Nile, and Indus--emphasizing recent findings and consensus as to their health and how human activities have brought these vulnerable ecosystems perilously close to or beyond the point of no return. [ABSTRACT FROM AUTHOR]

The Kangerlussuaq margin is well furrowed by icebergs. The margin favors furrow preservation since sedimentation on the continental shelf is low, at least since the last (Flakkerhuk) glaciation and because shelf depths are well below wave base. While furrows are common in the Kangerlussuaq region, the icebergs that created the larger furrows are mostly absent. “Fresh” iceberg furrows are observed only on the bathymetric highs surrounding the 750-m-deep Kangerlussuaq Trough. Bathymetric barriers around the trough prevent icebergs with deep keels from entering the trough. Enormous and weathered (older) furrows are located inside the protected trough, apparently formed at paleocalving ice sheet termini, when the Greenland Ice Sheet extended onto the shelf. In the deeper sections of Kangerlussuaq Trough, Holocene mud is presently burying these older iceberg furrows. Paleocalving ice sheet margins, possibly from Iceland, account for muted furrows found on the upper continental slope of East Greenland. No sediment has been deposited over these hardground sites since 14,750 ± 720 BP.

New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about $$15 \times 10^{6}$$tons) contribute about $$7 \times 10^{9}$$tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about $$13.5 \times 10^{9}$$tons annually; bedload and flood discharges may account for an additional $$1-2 \times 10^{9}$$tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.

During the Holocene sediment input from the Yellow River to the Yellow Sea and adjacent areas has amounted to about 3000 $$km^{3}$$. Detailed records from river gauging stations over the past 70 yrs, together with estimates based on earlier historic records, suggest that the river has contributed 2300 $$km^{3}$$of sediment during the last 2300 yrs, an average discharge of $$1 km^{3}/yr (1 X 10^{9} t/yr)$$. Prior to extensive agricultural use of the loess plateau in northern China (beginning about 200 BC), therefore, the river's load must have been about $$0.1 km^{3}/yr$$, an order of magnitude lower than that at present. A broad delta 20 m below sea level suggests that early Holocene rates were higher than the pre-agricultural average and therefore that middle Holocene loads were low.

The small mountainous rivers draining the large island of New Guinea (area = 800,000 km2) discharge approximately 1.7 × 109 t of sediment annually, about the same as the combined estimated loads of rivers draining North America. Sediment yields are particularly high for rivers that drain the high mountains of eastern New Guinea, where drainage basins are high and drainage areas small. Owing to the generally narrow (<10 km wide) shelf off eastern New Guinea, much of the fluvial sediment may escape directly to the adjacent deep sea, as evidenced by the presence of sediment-filled trenches. In contrast, many of the rivers draining the south—central part of the island discharge onto a fairly broad shelf, where much of the sediment presumably remains, perhaps as prograding mud wedges.

Calcareous tubes of the polychaete family Serpulidae are composed of magnesian calcite, aragonite, or mixtures of these two minerals. Mineralogy is controlled principally by generic factors, but may be modified-at least in some genera-by environmental factors. The percentage of aragonite in most serpulid tubes shows little or no correlation with water temperature. The $$MgCO_{3}$$concentration in the entirely calcitic tubes generally increases with increasing temperature, but the $$MgCO_{3}$$content in the calcite of tubes with mixed mineralogy appears to be dependent upon mineralogy-that is, as aragonite content increases, the magnesium carbonate within the calcite also increases so that the magnesium within the entire tube remains about constant. In tubes with more than 60% aragonite, however, the total magnesium content drops markedly.

Recrystallized planktonic limestones from two guyots in the North Atlantic are in oxygen-isotopic equilibrium with their present ambient waters, suggesting submarine lithification and recrystallization. The early stages of submarine lithification of carbonates may involve precipitation of, and replacement by, magnesium-rich calcite; with time this may invert to magnesium-poor calcite. This type of lithification probably requires very low rates of sediment accumulation.

A sea-level curve of the past 35,000 years for the Atlantic continental shelf of the United States is based on more than 80 radiocarbon dates, 15 of which are older than 15,000 years. Materials include shallow-water mollusks, oolites, coralline algae, beachrock, and salt-marsh peat. Sea level 30,000 to 35,000 years ago was near the present one. Subsequent glacier growth lowered sea level to about -130 meters 16,000 years ago. Holocene transgression probably began about 14,000 years ago, and continued rapidly to about 7000 years ago. Dates from most shelves of the world agree with this curve, suggesting that it is approximately the eustatic curve for the period.

The Kangerlussuaq margin is well furrowed by icebergs. The margin favors furrow preservation since sedimentation on the continental shelfis low, at least since the last (Flakkerhuk) glaciation and because shelf depths are well below wave base. While furrows are common in the Kangerlussuaq region, the icebergs that created the larger furrows are mostly absent. 'Fresh' iceberg furrows are observed only on the bathymetric highs surrounding the 750-m-deep Kangerlussuaq Trough. Bathymetric barriers around the trough prevent icebergs with deep keels from entering the trough. Enormous and weathered (older) furrows are located inside the protected trough, apparently formed at paleocalving ice sheet termini, when the Greenland Ice Sheet extended onto the shelf. In the deeper sections of Kangerlussuaq Trough, Holocene mud ispresently burying these older iceberg furrows. Paleocalving ice sheet margins, possibly from Iceland, account for muted furrows found on the upper continental slope of East Greenland. No sediment has been deposited over these hardground sites since 14,750 +/- 720 BP. [ABSTRACT FROM AUTHOR]