Carbonate sedimentation through the late Precambrian and Phanerozoic

The global sediment mass-age distribution indicates large variations in the rates of carbonate sedimentation through time. The largest mass of carbonate deposited during the entire history of the earth was produced during the Cambrian, possibly following on an episode of phosphogenesis in the Late Precambrian. A second major episode occurred during the Late Devonian, probably reflecting the invasion of land by plants that altered the rock-weathering and soil-forming regimes. Other lesser pulses of carbonate deposition occurred in the Late Permian, Triassic, and Cretaceous. A shift in the locus of carbonate deposition from shallow waters to the deep sea occurred during the Cretaceous

Understanding and modeling the sedimentary system

The sedimentary system involves processes that weather rocks and reduce them to soluble and fine-grained particulate components that can be transported. deposited, and transformed back into rock. !Jost of the processes can be observed today, but the present is an unusual episode in our planet's history. We live in a brief warm interglacial epi sode in an interval usually characterized by large mid-and high-latitude icc sheets and a much lower sea level. To complicate matters further, few measurements of process rates were made before the significant impacts of agriculture and the industrial revolution altered them. Consequently, the rates at which different processes operate over most of geologic time arc not well known. The objective of modeling sedimentary systems is to simplify these processes so that they can be described in mathematical terms. Successful models predict the results of weathering. erosion, transport, depositional and diagenetic processes and allow us to determine process rates from ancient deposits. Modeling can also suggest the kinds of geologic information that can be used for its validation

Sedimentological and geochemical trends resulting from the breakup of Pangaea

The breakup of Pangaea and formation of the Atlantic and Indian Oceans and the marginal seas has an important influence on the global geochemistry of sediments

Pliocene-Quaternary upwelling in the Southeastern Atlantic may reflect changes in water mass production

The sediments recovered at Deep Sea Drilling Project Sites 362 and 532 on Walvis Ridge Abutment Plateau and at Site 530 in the southeastern Angola Basin record long-term changes in the rates of upwelling. Deposition of opaline silica and organic carbon increased from latest Miocene to latest Pliocene then declined to present. The sediments display light-dark cycles. The dark cycles contain more terrigenous material and represent glacials. During the Late Miocene the productivity maxima were characteristic of glacial maxima in the Antarctic. Since the beginning ofthe Pliocene productivity maxima have occurred during interglacials. The most likely causes of these changes are: 1) desiccation and reflooding of the Mediterranean. The desiccation drew the ITCZ to its most northerly position. After reflooding the Mediterranean had a positive fresh-water balance until about 2.5 Ma, when it changed to its present negative balance and lagoonal circulation. The period during which productivity increased along the southwest African margin corresponds to the time when the Mediterranean had a positive fresh-water balance and estuarine circulation. During this time the Mediterranean supplied no intermediate water to the North Atlantic. The decline in productivity off southwest Africa corresponds to the time when lagoonal circulation developed in the Mediterranean and, as at present, its outflow forms a major intermediate water mass. During glacials the more dilute saline Mediterranean outflow resulted in the expansion of nutrient-poor North Atlantic Intermediate Water (NAIW) at a higher level in the ocean. The NAIW replaced AAIW in the South Atlantic during glacials. Upwelling along Southwest Africa may have increased as a result of increased wind stress, but the upwelled water was NAIW, and did not result in increased productivity. 2) growth of the Antarctic and Northern Hemisphere ice caps. During the Late Miocene growth of the Antarctic ice cap forced northward migration of the subtropical highs and Intertropical Convergence Zone (ITCZ). These changes in atmospheric circulation may have initiated productive upwelling over the Walvis Abutment Plateau. As Northern Hemisphere glaciation was initiated, the Earth changed from a unipolar to a bipolar glaciated state. This forced southward migration of the ITCZ and an increase in the intensity of the southeast trade winds. 3) closing of the Central American Straits. The resulting salinization of the North Atlantic forced increased production of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). The production of NADW resulted in nutrient export from the North Atlantic and development of the contrast between nutrient-rich southern and nutrient-poor northern intermediate and deep water masses. The combination of all these changes is probably responsible for the observed pattern of change in productivity. Hay and Brock's (1992) explanation of lessened productivity during glacials being due to upwelling of nutrient-poor NAIW rather than AAIW remains a viable hypothesis

The Role of Polar Deep Water Formation in Global Climate Change

The present ocean is thermally stratified. The waters become colder with depth, and the great mass of ocean deep water is near 2°C-almost the same as that of surface waters in the polar regions. The idea that the waters have a polar origin goes back to Benjamin Thompson (1800)(Count Rumford). He reasoned that the cold waters of the interior of the ocean must form by sinking in the polar regions and must drive poleward flow of surface waters. The idea was refined by Alexander von Humboldt(1814), who noted that the density of sinking cold polar waters must exceed the density of more saline waters in lower latitudes

Cretaceous paleoceanography

The modern ocean is comprised of four units: an equatorial belt shared by the two hemispheres, tropical subtropical anticyclonic gyres, mid-latitude belts of water with steep meridional temperature gradients, and polar oceans characterized by cyclonic gyres. These units are separated by lines of convergence, or fronts: The Early Cretaceous ocean closely resembled the modern ocean. The developing Atlantic was analogous to the modern Mediterranean and served as an Intermediate Water source for the Pacific. Because sea-ice formed seasonally in the Early Cretaceous polar seas, deep water formation probably took place largely in the polar region. In the Late Cretaceous, the high latitudes were warm and deep water moved from the equatorial region toward the poles, enhancing the ocean's capacity to transport heat poleward. The contrast between surface gyre waters and intermediate waters was less. -from Autho

Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life

A compilation of data on volumes and masses of evaporite deposits is used as the basis for reconstruction of the salinity of the ocean in the past. Chloride is tracked as the only ion essentially restricted to the ocean, and past salinities are calculated from reconstructed chlorine content of the ocean. Models for ocean salinity through the Phanerozoic are developed using maximal and minimal estimates of the volumes of existing evaporite deposits, and using constant and declining volumes of ocean water through the Phanerozoic. We conclude that there have been significant changes in the mean salinity of the ocean accompanying a general decline throughout the Phanerozoic. The greatest changes are related to major extractions of salt into the young ocean basins which developed during the Mesozoic as Pangaea broke apart. Unfortunately, the sizes of these salt deposits are also the least well known. The last major extractions of salt from the ocean occurred during the Miocene, shortly after the large scale extraction of water from the ocean to form the ice cap of Antarctica. However, these two modifications of the masses of H2O and salt in the ocean followed in sequence and did not cancel each other out. Accordingly, salinities during the Early Miocene were between 37‰ and 39‰. The Mesozoic was a time of generally declining salinity associated with the deep sea salt extractions of the North Atlantic and Gulf of Mexico (Middle to Late Jurassic) and South Atlantic (Early Cretaceous). The earliest of the major extractions of the Phanerozoic occurred during the Permian. There were few large extractions of salt during the earlier Palaeozoic. The models suggest that this was a time of relatively stable but slowly increasing salinities ranging through the upper 40‰'s into the lower 50‰'s. Higher salinities for the world ocean have profound consequences for the thermohaline circulation of the ocean in the past. In the modern ocean, with an average salinity of about 34.7‰, the density of water is only very slightly affected by cooling as it approaches the freezing point. Consequently, salinization through sea-ice formation or evaporation is usually required to make water dense enough to sink into the ocean interior. At salinities above about 40‰ water continues to become more dense as it approaches the freezing point, and salinization is not required. The energy-consuming phase changes involved in sea-ice formation and evaporation would not be required for vertical circulation in the ocean. The hypothesized major declines in salinity correspond closely to the evolution of both planktonic foraminifera and calcareous nannoplankton. Both groups were restricted to shelf regions in the Jurassic and early Cretaceous, but spread into the open ocean in the mid-Cretaceous. Their availability to inhabit the open ocean may be directly related to the decline in salinity. The Permian extraction may have created stress for marine organisms and may have been a factor contributing to the end-Permian extinction. The modeling also suggests that there was a major salinity decline from the Late Precambrian to the Cambrian, and it is tempting to speculate that this may have been a factor in the Cambrian explosion of life