Magazine article Oceanus

Terrigenous Sediments in the Pelagic Realm

Magazine article Oceanus

Terrigenous Sediments in the Pelagic Realm

Article excerpt

The composition, mass accumulation rate, and grain size of the terrigenous component of deep-sea sediment provide records of both the sediment's continental source region and of transport and depositional processes. By volume, most terrigenous material arrives in the deep ocean through the deposition of turbidites (see "Turbidite Sedimentation," page 107). Here I will review the other three pertinent processes and outline how 25 years of ocean drilling has allowed marine geologists to understand the earth history thus recorded. The three processes provide the following kinds of sediment to the deep sea:

* ice-rafted debris, which gives direct physical evidence of glaciers at sea level;

* aeolian (wind-borne) dust, which offers information about the climate of the dust-source region and the intensity of the transporting winds; and

* hemipelagic muds, which record continental erosion and runoff in their flux data.

There is no way to obtain long, relatively continuous records of these processes other than ocean drilling. The resulting cores contain information that spans extended time periods so that geologists may track global change through many tens of millions of years. The hydraulic advanced piston core (APC) technology developed by the drilling program also permits recovery of undisturbed cores containing very high-resolution sequences of the climate cycles of the past few hundred thousand years. Finally, a quarter of a century of ocean drilling has resulted in nearly global coverage; samples are available from most parts of the world's oceans with the exception of the Arctic and the central Pacific south of about 20 |degrees~ S latitude.

Ice-Rafted Debris

The geological history of glaciation has been a subject of lively debate ever since Swiss naturalist and geologist Louis Agassiz (1807-1873) convinced the scientific public that his idea of vast continental glaciers was correct. Discussions of the timing of glacial onset centered first on the Northern Hemisphere, and then the Southern. Early in the history of DSDP, Legs 12 to the North Atlantic and 18 and 19 to the North Pacific had among their major objectives the determination of the timing of Northern Hemisphere glaciation, especially the age of glacial onset. Cores from all three of these cruises dearly showed that ice-rafted debris became an important component of the sediment at a time then estimated to be in the middle to late Pliocene. Later, North Atlantic Legs 38 and 49 confirmed the original results of Leg 12.

The ice-rafted debris stratigraphy was reasonably dear in these regions, but use of the hydraulic piston corer in the late 1970s along with improved resolution of the biostratigraphy, oxygen-isotope stratigraphy, and magnetic-reversal stratigraphy were needed before the timing of glacial onset could be determined precisely. These improved stratigraphies were provided for the North Atlantic by the scientists of Leg 81, and by the mid 1980s it became dear that Northern Hemisphere ice rafting began in both the North Atlantic and North Pacific almost exactly at the time of the Matuyama-Gauss magnetic reversal, recently dated at 2.6 million years ago.

The details of high northern latitude glaciation were an important objective of DSDP Leg 94 and ODP Legs 104, 105, 151, and 152 to the North Atlantic and DSDP Leg 86 and ODP Leg 145 to the North Pacific. In addition to further defining the Plio-Pleistocene glaciation 0 to 2.6 million years ago, these cruises found evidence for a latest-Miocene to earliest-Pliocene ice advance: 4- to 6-million-year-old glacial dropstones have been recovered from both the North Atlantic and North Pacific oceans.

The drilling history of the high southern latitudes is similar. Legs 28 and 35 recovered ice-rafted debris as old as the Oligocene with large numbers of such grains occurring in Miocene and younger sediments. ODP has made high southern latitudes a special target. …

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