The Amazon River is, by far, the world's largest in its volume of fresh-water discharge. The basin is also the most bio-diverse on Earth. Climate models predict vastly different outcomes for the basin over the next century, from unchanged or wetter than today, to much drier, savannah or even desert conditions. Such different climate outcomes of course imply very different ecological consequences Which prediction is correct? How can we choose? Are there any clues about the future of the Amazon basin contained in the geologic record of its past?
We now know that during the ice ages (the Pleistocene) through the last 10,000 years (the Holocene), there were large changes in the climate and hydrology of the Amazon basin. These are perhaps best expressed in new (mostly unpublished) stable oxygen isotopic records of stalagmites from a few scattered caverns. What caused these large climate changes of the past? Is similar variability possible for the future of the Amazon? Of course, the Amazon basin is huge (comparable to the area of the continental US), and has a spatially variable climate, thus, more than a few records are needed to characterize this variety.
The Amazon River also transports huge volumes of suspended sediment, much of it derived from the high Andes to the west, less from the central and eastern lowlands. This sediment contains mineral matter and biotic remains including pollen. Although part of the sediment is trapped in the vast flood plain of the lower Amazon, most eventually arrives at the river mouth. Here, at the Atlantic margin of Brazil, the Amazon forms a freshwater plume that is entrained by the North Brazil current and advects northwestward along the coast toward French Guiana. A portion of the Amazon-derived sediment is deposited on the inner continental shelf northwestward of the river mouth and a portion of the sediment bypasses the shelf and is deposited farther offshore along the Amazon continental slope and the deepwater fan. During past glacial periods, when thick ice sheets covered large parts of the northern hemisphere land area, sea level fell and continental shelves were exposed worldwide. During those times, the Amazon river cut across the Brazilian shelf to deposit its sediments directly into the head of the Amazon fan.
Our plan on this cruise is to reconstruct past climates of the Amazon basin region from the analysis of sediment cores taken from the Brazilian continental slope offshore of the mouth of the Amazon River. Our aim is to recover a continuous sedimentary record of past climate that extends at least 30,000 years into the past, thus covering the last global glacial maximum (ca. 25,000 years ago). Using these same cores we will also reconstruct the sea-surface temperature (SST) of the western equatorial Atlantic. We plan to take a series of cores north and south of the equator. These cores and subsequent analyses will allow us to address the hypothesis that the North-South SST gradient of the western equatorial Atlantic plays a major role in forcing precipitation in the adjacent Amazon basin.
Our fieldwork will consist of collecting large-diameter gravity cores, long piston cores using the Knorr's new long coring facility, and near-surface multi cores and box cores. These will be complemented with conductivity-temperature-depth (CTD) casts and hydrocasts to learn about the water physical properties and chemical composition. These complementary studies will aid in calibrating our geochemical proxies.
A second aspect of our work will concentrate on regions of compressional folding and faulting near the base of the depositional fan. This is an area of abundant gas hydrates, mega-slumps, fluid flow, and, what appear to be, deep-water carbonate mounds. We plan to take gravity cores for pore-fluid and gas analyses to determine whether the hydrates are of thermogenic or biogenic origin. We will also smash gravity cores into the carbonate mounds to attempt recovery of some of the lithified substrate.
The geology of the whole field area is known to be extraordinarily complex and we will rely upon high-resolution seismic reflection profiling using a chirp source to ensure continuous and conformable sediment deposition at the coring sites. We will also construct SEABEAM mapping mosaics of selected portions of the field region. On all cores we will undertake instrumental core logging and visual core description. The analytical work to be undertaken in our home laboratories on these cores will include clay mineralogy to elucidate terrestrial sediment sources; paired analysis of planktonic foraminifer minor element ratios (Mg/Ca and Ba/Ca) and stable oxygen isotopic ratios (δ18O) to determine SST and sea-surface salinity (SSS); alkenone undersaturation index and stable hydrogen isotopic ratios (δD) of algal lipids as complementary measures of SST and SSS; abundances, δD, and stable carbon isotopic ratios (δ13C) of terrigenous plant waxes to assess continental hydrology and vegetation. A complete pollen time series will also be generated to determine whether past changes in major vegetation type were associated with changes in climate. Age models will be constructed for both target cores using AMS 14C measurements of mono-specific samples of planktonic foraminifers.
The box below outlines the field area of the cruise.
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| Field area of the cruise. Contour interval is 10 m on the continental shelf (greater than 120 m water depth) and 200 m in deeper regions |
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