The NSF and the geosciences community: Rotating program officers

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95447/1/eost7407.pd

Aspects of atmospheric circulation: the late Pleistocene (0-950,000 yr) record of eolian deposition in the pacific ocean

The eolian component of pelagic sediments provides a proxy record of atmospheric circulation intensity and of dust transport. Examination of those records indicates that the atmosphere responds to orbital forcing but that during the past 875,000 years the, variability in atmospheric circulation is at much shorter periods than the 100,000-year variation in ice volume. The mid-Brunhes climatic event is well characterized in the eolian grain-size records and in records of sea-surface phenomena; it is not seen in proxy indicators of either deep-water or ice-volume variability. The increase in the amplitude of paeloclimatic variability that denotes the early/late Pleistocene transition occurs suddenly about 875,000 years ago in the eolian grain-size record, which is about 20,000 to 25,000 years earlier than the transition in the deep sea CaCO3-dissolution record or in the [delta]18O proxy record of ice volume.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28534/1/0000332.pd

Analysis of a fast-spreading rise crest: The East Pacific Rise, 9° to 12° south

The axis of the East Pacific Rise is defined by a topographic block about 15 km wide and 300 to 350 m high which is flanked by abyssal hills 100 to 200 m high and 3 to 5 km wide. These hills often are tilted such that their steep slopes face the axis. An empirical model explaining these features combines axial extrusion to form the central block and rotational faulting to lower the shoulders of the axial block to the regional depth and tilt them outward.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43191/1/11001_2004_Article_BF00426250.pd

Tectonics of the East Pacific Rise, 5⁰ to 12⁰S

The portion of the East Pacific Rise (EPR) between 5° and 12°S can be subdivided into three areas by fracture zones that offset the rise axis at about 6.5°S and 9°S. In all three regions, the spreading axis of the EPR is defined by a topographic block 300 to 350 m high and about 15 km wide. The axial block commonly has subordinate peaks or shoulders on each flank that vary up to 5 km wide. Analysis of linear magnetic anomalies in the southern of the three areas suggests that sea-floor spreading has been occurring at rates of 80 mm/y to the west and 77 mm/y to the east during the last 1.7 million years (my). The rise axis is offset in two places about 10 km. These displacements are interpreted to be the result of discrete jumps of the site of spreading activity. Reconnaissance data from the east flank of the EPR indicate that spreading activity on this portion of the rise began just over 8 my ago when the site of crustal accretion jumped westward from the now dormant Galapagos Rise. About 4.6 my ago the section of rise crest between approximately 10.5°S and the 9°S fracture zone jumped westward a distance of at least 10 km, isolating the pre-existing axial block. By 1.7 my ago, this axial offset no longer existed, implying a relatively westward migration of the rise axis south of 10.5° during the period from 4.6 to 1.7 my ago. The two offsets of the present axis occurred between 1.6 and 0.9 my ago and 0.7 my ago and the present in the north and south, respectively. The formation of the axial block, its associated shoulders and the abyssal topography, apparently consisting of tilted fault blocks with steep sides facing the axis, can be explained by an empirical model. This model combines extrusion of basalt along the spreading axis to form the axial block and rotational faulting to form the shoulders of the block and then lower the shoulders to merge with the abyssal topography of the uppermost rise flanks. In the region north of the 6.5°S fracture zone, seismic activity on the rise crest, combined with topographic information, suggests that the EPR axis is unstable here and that it is in the process of reorienting to a more stable regional trend. Identification and mapping of magnetic anomalies in the vicinity of 6°S reveal a spreading rate of about 77 mm/y, an episode of oblique spreading that occurred 1.7 my ago, and a 10 km westward jump of the axis which occurred since 0.7 my ago. The fracture zone at 6.5°S offsets the EPR axis 55 km right laterally. Both the bathymetric and magnetic expression of this fracture zone are markedly reduced a few tens of kilometers away from its active portion, implying that some change in the processes forming the fracture zone occurred about 0.3 my ago. Between the fracture zones at 6.5° and 9°S, two deactivated segments of the rise axis, each about 40 km long, occur 90 and 65 km west of the currently active axis. These inactive axial segments were isolated by eastward jumps of the spreading center. The more northerly jump, along 6.5°S, occurred 0.3 my ago and spanned 68 km. The other, along 7°S, covered a similar distance and is either just completed or still going on. Reconnaissance data from this part of the EPR suggests that sea-floor spreading began here about 6.5 my ago and has been continuing at a rate of approximately 78 mm/y

Late Tertiary history of hydrothermal deposition at the East Pacific Rise, 19°S: Correlation to volcano‐tectonic events

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95132/1/grl3614.pd

Neogene history of the south Pacific tradewinds: Evidence for hemispherical asymmetry of atmospheric circulation

Eolian dust grains extracted from southeast Pacific pelagic sediments provide a Neogene record of southern hemisphere atmospheric processes. The mass accumulation rate of dust has been low and generally constant at 1-4 mg/cm2 103 y since the late Oligocene. Eolian grainsize data show an increase from 8.2[phi] (3.40 [mu]) in older sediments to 7.2[phi] (6.8o [mu]) in younger material. This shift occurred about 10.5 m.y. ago and reflects a significant increase in the intensity of atmospheric circulation then. There is no obvious response to the onset of northern hemisphere glaciation 2.5 m.y. ago in these eolian records from the South Pacific. Comparison with existing information from the northern hemisphere suggests that the southern hemisphere has had more intense atmospheric circulation throughout the Neogene and that this asymmetry reached its greatest extent between 10.5 and 2.5 Ma.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26165/1/0000242.pd

Eastern Pacific spreading rate fluctuation and its relation to Pacific area volcanic episodes

Sea-floor spreading rates from four locations along the Nazca-Pacific plate boundary and one along the Juan de Fuca-Pacific plate boundary show variations over the past 2.4 m.y., with decreasing rates prior to the Jaramillo to Olduvai time interval (0.92-1.73 m.y. ago) and increasing rates since then. Other Pacific area volcanic phenomena in mid-plate and convergent-boundary settings also show minima about 1.3-1.5 m.y. ago and a maximum at present and another maximum about 5 m.y. ago: extrusion rates along the Hawaiian Ridge; volcanic episodes associated with calc-alkalic provinces of western Oregon and Central America; temporal variations in the SiO2 content of Aleutian ash layers; and the number of deep-sea ash layers. These phenomena may fluctuate in response to changing spreading rates. During times of more rapid spreading increased shear and melting along lithospheric boundaries may occasion increased volcanic activity, whereas during times of less rapid spreading volcanic activity may be less intense.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23637/1/0000601.pd

Dry bulk density and CaCO3 relationships in upper Quaternary sediments of the eastern equatorial Pacific

Results from 1110 paired CaCO3 and bulk density measurements from cores raised from the eastern equatorial Pacific permit formulation of well constrained CaCO3-DBD relationships for that region. The cores lie along a N-S transect at 110[deg]W from 10[deg]N to 3[deg]S underneath the different currents of the equatorial current system and along an E-W transect from 110[deg] to 90[deg]W, at approximately 3[deg]S. Two distinct, crescent-shaped dry bulk density-CaCO3 relationships are observed. For equal CaCO3 percentages, sediments from those sites at 110[deg]W, which are situated in the high productivity zone, have lower dry bulk density. Cores raised from closer to shore have relatively greater DBD.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31363/1/0000275.pd

Late Cenozoic changes in atmospheric circulation deduced from North Pacific eolian sediments

Isolation and analysis of the eolian component of late Cenozoic pelagic sediments from the North Pacific provides direct information concerning changes in atmospheric circulation. A 50% increase in intensity of both the prevailing westerlies and the tradewinds coincides with increasing pole-to-equator temperature gradients resulting from the onset of northern hemisphere glaciation. At the same time, the mass flux of dust from continents to the North Pacific increased by a factor of 4.5, apparently reflecting significantly increased continental aridity associated with the late Cenozoic glacial ages.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23873/1/0000112.pd

Late Cretaceous and Paleogene tectonic evolution of the North Pacific Ocean

The Late Cretaceous history of the northern Pacific Ocean has not been adequately deciphered, largely because a major plate reorganization occurred during the Cretaceous magnetic quiet interval. Using primary data to reconstruct plate motions from fracture zone trends and Late Cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region, especially the Emperor and Chinook troughs. The primary event in our reconstruction is the subduction of the old northwest Pacific triple junction. New relative plate motions imposed by formation of convergent boundaries along both the northern Pacific and Farallon plates caused the Farallon plate to crack. This subdivision occurred 82 m.y. ago and resulted in the formation of the Kula and Chinook plates. The Chinook plate was bounded on the north by the Chinook-Kula ridge, the western arm of the Great Magnetic Bight, on the west by the southern Emperor trough, a slowly spreading rift valley, on the south by the Mendocino transform, and on the east by the Chinook-Farallon ridge. The northern Emperor trough formed the initial western boundary of the Kula plate. From 82 m.y. ago to 50 m.y. ago this configuration was stable: the Chinook plate expanded, the Chinook-Kula ridge migrated north, and the Kula plate was subducted. With the subduction of this ridge, the Chinook plate adhered to the Pacific plate and the Pacific-Farallon ridge became the north-south-trending feature reflected by anomalies 22 and younger.This reconstruction obviates the awkward Late Cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region. To achieve closure of vector triangles representing the relative Late Cretaceous plate motions some extension must occur, presumably across the eastern Mendocino fracture zone, between the northern and southern portions of the Farallon plate. These two plates apparently were independent units between 105 and 50 m.y. ago; the initiation and cessation of rapid North Farallon-North American convergence 82 and 50 m.y. ago may have determined the nature and duration of the Laramide orogeny.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25097/1/0000529.pd