Margaret Leinen

Paleoclimatology and paleometeorology : modern and past patterns of global atmospheric transport

Section 1. Dust Formation, Injection, and Continental Aridity.- 1. Processes of fine particle formation, dust source regions, and climatic changes.- 2. Formation of winderodible aggregates for salty soils and soils with less than 50% sand composition in natural terrestrial environments.- 3. Dust and climate in the American Southwest.- 4. Climatic controls on the frequency, magnitude and distribution of dust storms: examples from India/Pakistan, Mauritania and Mongolia.- 5. Emission of fine-grained particulates from desert soils.- 6. Relationships between the characteristics of soils, the wind energy and dust near the ground, in the western sandsea (N.W. Sahara).- 7. Aeolian dust transport in West Africa.- Section 2. Dust Transport and Transport Modeling.- 1. Modeling long-range transport using trajectory techniques.- 2. Present transport and deposition patterns of African dusts to the North-western Mediterranean.- 3. Desert dust and climate: an investigation using an atmospheric general circulation model.- Section 3. Dust Composition and Factors Controlling it: Evidence from Aerosols and Sediments.- 1. Eolian dust over the Mediterranean and their contribution to the present sedimentation.- 2. Eolian dust collected in springtime (1979 and 1984 Years) at the seawater-air interface of the Northern Red Sea.- 3. Desert aerosol: characteristics and effects on climate.- 4. Local, proximal and distal Saharan dusts: characterization and contribution to the sedimentation.- 5. Atmospheric mineral dust - properties and source markers.- 6. Nd and Sr isotopes as tracers of wind transport: Atlantic aerosols and surface sediments.- 7. Wind-borne deposits in the Northwestern Indian Ocean: record of Holocene sediments versus modern satellite data.- 8. Aeolian-derived higher plant lipids in the marine sedimentary record: links with palaeoclimate.- 9. Lithogenic sediment on Arctic pack ice: potential aeolian flux and contribution to deep sea sediment.- Section 4. Modeling Atmospheric Circulation in the Past.- 1. Past and present oceanic energy balance patterns.- 2. Possible effects of orbital variations on past sources and transports of eolian material: estimates from general circulation model experiments.- Section 5. Inferences from the Sedimentary Record: Loess, Ice Cores, and Other Land Evidence.- 1. Climatic changes in Israel during historical times and their impact on hydrological, pedological and socio-economic systems.- 2. The mineral-dust record in a highaltitude Alpine Glacier (Colle Gnifetti, Swiss Alps).- 3. Fallout of Saharan dust in the Northwestern Mediterranean Region.- 4. Late Quaternary climatic changes in the African Rain Forest: forest refugia and the major role of sea-surface temperature variations.- 5. The regionalization of climatic change in Western North America.- 6. Interglacial environments in presently Hyperarid Sahara: palaeoclimatic implications.- Section 6. Report of the Group Studying: Inferences from the Marine Sedimentary Record.- 1. Northeast Asian climatic change over the last 140,000 years inferred from pollen in marine cores taken off the Pacific Coast of Japan.- 2. The Late Quaternary record of atmospheric transport to the Northwest Pacific from Asia.- 3. Variations of the NW African trade wind regime during the last 140,000 years: changes in pollen flux evidenced by marine sediment records.- 4. Deflation and humidity during the last 700 ka in NW Africa from the marine record.- 5. Palynology of the last 680,000 years of ODP Site 658 (off NW Africa): fluctuations in paleowind systems.- 6. Pliocene history of South Saharan/Sahelian aridity: record of freshwater diatoms (Genus Melosira) and opal phytoliths, ODP Sites 662 and 664.- 7. Cenozoic climatic variation recorded by quartz and clay minerals in North Pacific sediments.- 8. Geologic record of atmospheric circulation on tectonic time scales.- Section 7. Reports and Recommendations of Working Groups.- 1. Dust Formation, Injection, and Continental Aridity.- 2. Dust Transport and Transport Modeling.- 3. Dust Composition and Factors Controlling IT: Evidence from Aerosols and Sediments.- 4. Modeling Atmospheric Circulation in the Past.- 5. Inferences from the Sedimentary Record: Loess, Ice Cores, and other Land Evidence.- 6. Report of the Group Studying: Inferences from the Marine Sedimentary Record.

Geologic Approach to the Long-Term History of Atmospheric Circulation

Eolian dust preserved in deep-sea pelagic sediments can be used as a proxy indicator of paleoclimate. Analyses of the particle size, composition, and mass accumulation rate of dust grains provide independent evidence of the intensity of atmospheric circulation and the availability of material in the eolian source region. These data provide information on atmospheric circulation and on the climate of the source area at time scales ranging from 103 to 108 years and have the potential to test computer-generated global circulation models.

SCAVENGED EXCESS ALUMINUM AND ITS RELATIONSHIP TO BULK TITANIUM IN BIOGENIC SEDIMENT FROM THE CENTRAL EQUATORIAL PACIFIC OCEAN

We present results from chemical analyses of Al and Ti in surface sediment sampled along two cross-Equator latitudinal transects at 135W and 140W in the central equatorial Pacific Ocean. Although traditionally both Al and Ti are considered to reside exclusively within terrigenous phases in marine sediment, these sediments present extremely high Al/Ti ratios that are several times that of average shale and other potential crustal sources. A sharp maximum in Al/Ti is observed slightly south of the Equator, where sedimentary bulk accumulation rate (BAR) is also highest (reflecting elevated productivity in the overlying water caused by surface water divergence). Bulk Al/Ti decreases sharply away (±2° latitude) to near crustal values at ∼4° north and south. The latitudinal profiles of Al/Ti are entirely unrelated to the concentration of the biogenic components as well as to the absolute accumulation of Al and Ti. These results indicate the presence of a significant scavenged component of Al sourced directly from seawater during particle settling. The data from the two transects analytically and oceanographically confirms our earlier work that was based on the single 135W transect. Calculations of Alexcess indicate that the highest Al/Ti ratios correspond to ∼50% of the total Al being unsupported by the small amount of terrigenous phases present. These results are consistent with previous and ongoing studies of biogenic sediment, suspended particulate matter, and sediment trap material. Quantitative use of Al as an index of terrigenous material may, therefore, lead to an overestimation, by a factor of two, of the true terrigenous load in marine sediment, sedimentary rock, and settling particles. Because bulk Al/Ti appears to respond to sedimentary BAR, which in biogenic regimes is linked to surface water productivity, downcore records of Al/Ti in biogenic sediment may track productivity changes through time. Such Al/Ti proxy records may be applicable in sediment of all ages, unlike radionuclide tracers which are limited by radioactive decay to use over the past hundreds of kyr.

Chemical transport to the seafloor of the equatorial Pacific Ocean across a latitudinal transect at 135°W: Tracking sedimentary major, trace, and rare earth element fluxes at the Equator and the Intertropical Convergence Zone

We have analyzed the major, trace, and rare earth element composition of surface sediments collected from a transect across the Equator at 135°W longitude in the Pacific Ocean. Comparing the behavior of this suite of elements to the CaCO3, opal, and Corg fluxes (which record sharp maxima at the Equator, previously documented at the same sampling stations) enables us to assess the relative significance of the various pathways by which trace elements are transported to the equatorial Pacific seafloor. The 1. (1) high biogenic source at the Equator, associated with equatorial divergence of surface water and upwelling of nutrient-rich water, and 2. (2) high aluminosilicate flux at 4°N, associated with increased terrigenous input from elevated rainfall at the Intertropical Convergence Zone (ITCZ) of the tradewinds, are the two most important fluxes with which elemental transport is affiliated. The biogenic flux at the Equator transports Ca and Sr structurally bound to carbonate tests and Mn primarily as an adsorbed component. Trace elements such as Cr, As, Pb, and the REEs are also influenced by the biogenic flux at the Equator, although this affiliation is not regionally dominant. Normative calculations suggest that extremely large fluxes of Ba and P at the Equator are carried by only small proportions of barite and apatite phases. The high terrigenous flux at the ITCZ has a profound effect on chemical transport to the seafloor, with elemental fluxes increasing tremendously and in parallel with Ti. Normative calculations, however, indicate that these fluxes are far in excess of what can be supplied by lattice-bound terrigenous phases. The accumulation of Ba is greater than is affiliated with biogenic transport at the Equator, while the P flux at the ITCZ is only 10% less than at the Equator. This challenges the common view that Ba and P are essentially exclusively associated with biogenic fluxes. Many other elements (including Mn, Pb, As, and REEs) also record greater accumulation beneath the ITCZ than at the Equator. Thus, adsorptive scavenging by terrigenous paniculate matter, or phases intimately associated with them, appears to be an extremely important process regulating elemental transport to the equatorial Pacific seafloor. These findings emphasize the role of vertical transport to the sediment, and provide additional constraints on the paleochemical use of trace elements to track biogenic and terrigenous fluxes.

Biogenic flux of Al to sediment in the central equatorial Pacific Ocean: Evidence for increased productivity during glacial periods

We examined the flux of Al to sediment accumulating beneath the zone of elevated productivity in the central equatorial Pacific Ocean, along a surface sediment transect at 135°W as well as downcore for a 650 kyr record at 1.3°N, 133.6°W. Across the surface transect, a pronounced, broadly equatorially symmetric increase in Al accumulation is observed, relative to Ti, with Al/Ti ratios reaching values 3–4 times that of potential detrital sources. The profile parallels biogenic accumulation and the modeled flux of particulate 234Th, suggesting rapid and preferential adsorptive removal of Al from seawater by settling biogenic particles. Normative calculations confirm that most Al is unsupported by the terrigenous fraction. The observed distributions are consistent with previous observations of the relative and absolute behavior of Al and Ti in seawater, and we can construct a reasonable mass balance between the amount of seawater-sourced Al retained in the sediment and the amount of seawater Al available in the overlying column. The close tie between Al/Ti and biogenic accumulation (as opposed to concentration) emphasizes that biogenic sedimentary Al/Ti responds to removal-transport phenomena and not bulk sediment composition. Thus, in these sediments dominated by the biogenic component, the bulk Al/Ti ratio reflects biogenic particle flux, and by extension, productivity of the overlying seawater. The downcore profile of Al/Ti at 1.3°N displays marked increases during glacial episodes, similar to that observed across the surface transect, from a background value near Al/Ti of average upper crust. The excursions in Al/Ti are stratigraphically coincident with maxima in both bulk and CaCO3 accumulation and the excess Al appears to not be preferentially affiliated with opaline or organic phases. Consistent with the similar behavioral removal of Al and 234Th, the latter of which responds to the total particle flux, the Al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits. These results collectively indicate that (1) Al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater. Therefore, the common tenet that Al is dominantly associated with terrestrial particulate matter, and the subsequent use of Al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment, needs to be reevaluated. (2) The Al/Ti ratio in biogenic sediment can be used to trace the productivity of the overlying water, providing a powerful new paleochemical tool to investigate oceanic response to climatic variation. (3) The close correlation between the Al/Ti productivity signal and carbonate maxima downcore at 1.3°N suggests that the sedimentary carbonate maxima in the central equatorial Pacific Ocean record increased productivity during glacial episodes.

Distribution of biogenic silica and quartz in recent deep-sea sediments

All available quartz and biogenic silica concentrations from deep-sea surface sediments were intercalibrated, plotted, and contoured on a calcium-carbonate-free basis. The maps show highest concentrations of biogenic silica (opal) along the west African coast, along equatorial divergences in all oceans, and at the Polar Front in the southern Indian Ocean. These are all areas where upwelling is strong and there is high biological productivity. Quartz in pelagic sediments deposited far from land is generally eolian in origin. Its distribution reflects dominant wind systems in the Pacific, but in much of the Atlantic and Indian oceans the distribution pattern is strongly modified by turbidite deposition and bottom current processes.

Asian aridity and the zonal westerlies: Late Pleistocene and Holocene record of eolian deposition in the northwest Pacific Ocean

Rea, D. K. and Leinen, M., 1988. Asian aridity and the zonal westerlies: Late Pleistocene and Holocene record of eolian deposition in the northwest Pacific Ocean. Palaeogeogr., Palaeoclimatol., Palaeoecol., 66:1 8. A 30,000-year record of eolian deposition in the northwestern Pacific Ocean provides a history of the aridity of the Asian source region and information on the changing latitude and intensity of the zonal westerlies. The position of the maximum in eolian flux to the deep sea remained unchanged at 38 ° to 40°N during this entire time period, but the dust flux record of aridity is greatest 6000 years ago at the Holocene climatic optimum. Eolian grainsize data indicate that the northern margin of the westerly jet stream retreated northward during the Holocene to a poleward extreme at about 6000 years ago and then moved back toward the equator. The change in wind intensity across this margin was continually reduced throughout the Holocene. These sorts of data enhance our understanding of the behavior of the atmosphere during times of climate change and can be used to test computer-generated models of past climatic conditions.

An objective technique for determining end-member compositions and for partitioning sediments according to their sources

Abstract A modification of Q-mode factor analysis provides an objective technique for end-member characterization of geologic mixtures. Factor analysis has not often been used to determine the actual composition of end-member sources in geologic mixtures, because transformations of the original data variables during the analysis result in negative factor scores for some variables and negative “concentrations” of some variables in the end-members. We use a non-orthogonal rotation of end-member vectors toward the mean vector to bring the end-members into the positive vector space. This technique is demonstrated on a large data set of elemental analyses of bulk sediment surface samples from the Nazca Plate in the southeast Pacific. These data were chosen because they had been studied in detail previously. Our technique identified five geologically reasonable end-members which represent detritus, siliceous biogenic sediment, hydrothermal sediment, hydrogenous or authigenic sediment, and a refractory residue left after the dissolution of biogenic debris. The number, composition, and distribution of the end-members are similar to those derived from previous extensive partitioning studies of the sediments.

Mineralogy of aeolian dust reaching the North Pacific Ocean: 1. Sampling and analysis

The mineral composition of the <2-μm (clay) and 2- to 20-μm (silt) size fractions of aerosol particles collected near the margins of the North Pacific Ocean was determined by X ray diffraction analysis. Although the same mineral phases are present in the two size fractions, there are significant differences in the quartz, plagioclase, and kaolinite concentrations. North Pacific aerosols are primarily composed of illite in both size fractions in addition to kaolinite in the <2-μm size class and quartz and plagioclase in the 2- to 20-μm size class. Quartz and plagioclase are present in significantly higher amounts in the 2- to 20-μm size fraction, while kaolinite is greater in the <2-μ size class. Western North Pacific aerosol particles contain significantly higher <2-μm quartz than samples collected in the east, and eastern North Pacific aerosols have significantly higher concentrations of <2-μm smectite than those from the west.

Sedimentary indicators of atmospheric activity in the Northern Hemisphere during the Cenozoic

Abstract The concentrations and accumulation rates of eolian phases and the mineralogy of sediments from a North Pacific core (LL44-GPC3) have been used to evaluate changes in eolian sedimentation and atmospheric activity during the Cenozoic. The quartz content, and clay mineralogy of the sediments reflect changing source areas for eolian material whereas the accumulation rates of eolian phases reflect climatic variation and wind intensity. Eolian sedimentation decreased from 70 to 50 m.y. ago at the site as it moved north out of the equatorial Pacific where it was supplied with eolian debris from North America and Africa. Low rates of eolian sediment accumulation 50 to 25 m.y. ago reflect the temperate, humid environment of the early Tertiary and the lack of vigorous atmospheric circulation at that time. From 25 to 7 m.y. ago eolian accumulation increased as the site moved into the influence of the westerlies. From 7 to 3 m.y. ago there was a more rapid increase in accumulation of eolian material at the site which was not accompanied by changes in mineralogy or quartz content and was, therefore, related to climatic change and more vigorous atmospheric circulation. About 2.5 m.y. ago a dramatic increase in eolian sedimentation accompanied the onset of dry Northern Hemisphere glaciation.