/pdf/global-continental-and-ocean-basin-reconstructions-since-200-huopwd2ldv.pdf

Global continental and ocean basin reconstructions since 200 Ma

Tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale, making use of improved chronostratigraphic control and additional Oligocene isotope data Synchronous changes in both benthic and planktonic δ18O values which occurred in the Oligocene to Miocene (36–52 Ma) are interpreted, in part, to represent ice growth and decay The inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records This association is consistent with a link between Oligocene to Miocene erosional events and rapid (>15 m/my) glacioeustatic lowerings of about 50 m High benthic foraminiferal δ18O values suggest the presence of continental ice sheets during much of the Oligocene to Recent (36–0 Ma) Substantially ice-free conditions probably existed throughout the Paleocene and Eocene (66–36 Ma) The mechanisms and rates of sea level change apparently were different between the early and late Tertiary, with glacioeustatic changes restricted to the past 36 my Pre-Oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes We apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands, stimulating slope failure and submarine erosion The different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late Tertiary on passive continental margins This speculation needs to be confirmed by examination of data from several passive margins

Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion

Oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on Antarctica since the earliest Oligocene (circa 35 Ma). The growth and decay of ice sheets during the Oligocene to modern “ice house world” caused glacioeustatic sea level changes. The early Eocene was an ice-free “greenhouse world,” but it is not clear if ice sheets existed during the middle to late Eocene “doubt house world.” Benthic foraminiferal δ18O records place limits on the history of glaciation, suggesting the presence of ice sheets at least intermittently since the earliest Oligocene. The best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic δ18O records. Although planktonic isotope records from the western equatorial regions are limited, subtropical planktonic foraminifera may also record such ice volume changes. It is difficult to apply these established principles to the Cenozoic δ18O record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events. We improved Oligocene to Miocene correlations of δ18O records and erected eight oxygen isotope zones (Oi1-Oi2, Mi1-Mi6). Benthic foraminiferal δ18O increases which are associated with the bases of Zones Oil (circa 35.8 Ma), Oi2 (circa 32.5 Ma), and Mil (circa 23.5 Ma) can be linked with δ18O increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near Antarctica. Our new correlations of middle Miocene benthic and western equatorial planktonic δ18O records show remarkable agreement in timing and amplitude. We interpret benthic-planktonic covariance to reflect substantial ice volume increases near the bases of Zones Mi2 (circa 16.1 Ma), Mi3 (circa 13.6 Ma), and possibly Mi5 (circa 11.3 Ma). Possible glacioeustatic lowerings are associated with the δ18O increases which culminated with the bases of Zone Mi4 (circa 12.6 Ma) and Mi6 (circa 9.6 Ma), although low-latitude planktonic δ18O records are required to test this. These inferred glacioeustatic lowerings can be linked to seismic and rock disconformities. For example, we link 12 Oligocene-early late Miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries (= inferred eustatic lowerings) of Haq et al. (1987).

Unlocking the Ice House: Oligocene‐Miocene oxygen isotopes, eustasy, and margin erosion

Around the globe, and in a variety of settings including active and inactive mountain belts, increases in sedimentation rates as well as in grain sizes of sediments were recorded at approximately 2-4 Myr ago, implying increased erosion rates. A change in climate represents the only process that is globally synchronous and can potentially account for the widespread increase in erosion and sedimentation, but no single process-like a lowering of sea levels or expanded glaciation-can explain increases in sedimentation in all environments, encompassing continental margins and interiors, and tropical as well as higher latitudes. We suggest that climate affected erosion mainly by the transition from a period of climate stability, in which landscapes had attained equilibrium configurations, to a time of frequent and abrupt changes in temperature, precipitation and vegetation, which prevented fluvial and glacial systems from establishing equilibrium states.

Increased sedimentation rates and grain sizes 2-4 Myr ago due to the influence of climate change on erosion rates.

The mineral component of pelagic sediment is brought to the deep sea by transport in the wind. Extraction and analysis of this dust allows estimation of the past aridity of the eolian source region, via flux determinations, and of the intensity of the transporting winds, from grain size data. These two parameters, the grain size and mass flux of dust, vary independently. There are three significant sources of modern dust, eastern and central Asia, northwest Africa, and Arabia, all in the northern hemisphere. As the rainfall associated with the Intertropical Convergence Zone is an effective barrier to southerly transport of dust, the northern hemisphere is an order of magnitude more “dusty” than is the southern, an asymmetry that has characterized most of the Cenozoic. Eolian flux data show that most of the northern hemisphere was more arid during glacial maxima, with 3 to 5 times as much dust transported during glacial stages than during interglacials; only northwestern South America varied in the opposite sense. The periodicity of Quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the Milankovitch cycles of orbital variability. Wind intensities vary on a shorter time scale than the general 100-kyr cycles of glaciation and general aridity. Eolian grain sizes display forcing at precessional (19 and 23 kyr), tilt (41 kyr), and at approximately 30 kyr periodicities. As a result the generalization that winds are uniformly stronger during glacial times is not valid. Whole-Cenozoic records show that the largest change in dust flux, an order of magnitude increase, occurred in the northern hemisphere and reflects continental drying associated with the late Pliocene onset of northern hemisphere glaciation. Southern hemisphere eolian records show no sign of paleoclimatic changes in the late Pliocene. The most important change in Cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the Paleocene-Eocene boundary. Before then, latest Cretaceous and Paleocene winds were essentially as strong as those of the late Cenozoic. This shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 Ma.

The paleoclimatic record provided by eolian deposition in the deep sea: The geologic history of wind

A record of Appalachian denudation in postrift Mesozoic and Cenozoic sedimentary deposits of the U.S. Middle Atlantic continental margin

Seismic profiles, drill core samples, and gravity data suggest that a complex impact crater ∼35.5 million years old and 90 kilometers in diameter is buried beneath the lower Chesapeake Bay. The breccia that fills the structure contains evidence of shock metamorphism, including impact melt breccias and multiple sets of planar deformation features (shock lamellae) in quartz and feldspar. The age of the crater and the composition of some breccia clasts are consistent with the Chesapeake Bay impact structure being the source of the North American tektites.

https://science.sciencemag.org/content/sci/271/5253/1263.full.pdf

Impact Origin of the Chesapeake Bay Structure and the Source of the North American Tektites

Ecologic atlas of benthic foraminifera of the Gulf of Mexico

New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A polymictic, late Eocene impact breccia, composed mainly of locally derived sedimentary debris (determined from four continuous cores), surrounds and partly fills the crater. Structural and sedimentary characteristics of the Chesapeake Bay crater closely resemble those of the Miocene Ries peakring crater in southern Germany. We speculate that the Chesapeake Bay crater is the source of the North American tektite strewn field.

C. Wylie Poag

Papers

A record of Appalachian denudation in postrift Mesozoic and Cenozoic sedimentary deposits of the U.S. Middle Atlantic continental margin

Impact Origin of the Chesapeake Bay Structure and the Source of the North American Tektites

Ecologic atlas of benthic foraminifera of the Gulf of Mexico

Meteoroid mayhem in Ole Virginny: source of the North American tektite strewn field

The Chesapeake Bay Crater : Geology and Geophysics of a Late Eocene Submarine Impact Structure