PP41A-0575 0800h
The effect of rising atmospheric oxygen on carbon and sulfur isotope anomalies recorded in the Neoproterozoic Johnnie Formation, Death Valley, USA
Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death Valley, California record a remarkable negative $\delta$$^{13}$C anomaly to a nadir of near -11$\permil$, accompanied by a dramatic rise in trace sulfate abundance ($>$500 ppm) and drop in carbonate associated sulfate $\delta$$^{34}$S values from 26.6$\permil$ to 15.8$\permil$. The carbonates, including the laterally extensive Johnnie Oolite, were deposited during marine flooding atop a sequence boundary best observed in cratonward sections. A similarly dramatic carbon isotope anomaly is recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian-Cambrian boundary in Oman, India, China, Australia, and Namibia. The cause of the carbon and sulfur isotopic excursions was likely associated with a rise in atmospheric O$_{2}$, which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, providing a source of $^{12}$C, $^{32}$S, and sulfate; and 2) the ventilation of the oceans. Large metazoan fossils (Ediacaran animals) first appear directly above this anomaly, suggesting that a critical threshold with respect to atmospheric O$_{2}$ had been crossed at this time. A negative $\delta$$^{13}$C excursion of similar magnitude occurs in overlying strata at the Precambrian-Cambrian boundary, which may reflect similar processes.
PP41A-0576 0800h
Mineral Surface Control of Organic Carbon Burial: Secular Rise of Clay Mineral Deposition in the Precambrian and the Rise of Oxygen
Accumulation of oxygen in the earth's atmosphere requires burial of organic matter in marine sediments. Today, the major mode of organic carbon burial is in association with detrital pedogenic clay minerals which serve to protect organic matter against biological oxidation during burial in marine sediments. The bulk of detrital clays that are ultimately deposited in marine sediments are formed in biologically active soils that require plant processes to retain water, concentrate weathering produced solutes, stablize soils, and provide an adsorptive media. At some point in Earth history before the colonization of land surfaces by plants and the formation of biotic soils, clay mineral surface limitation may have severely reduced the preservation potential of organic carbon during burial. An important consequence of this would have also been a reduced flux of oxygen to the atmosphere because organic carbon and oxygen release are coupled. Multiple independent lines of evidence indicate a significant change in continental weathering and pedogenic clay mineral formation and establishment of the `clay factory' that coincides with colonization of land surfaces by primitive plant like organisms in the late Precambrian. The enhanced burial efficiency that would have accompanied the shift to the modern mode of detrital pedogenic clay hosted carbon burial would have driven an increase in oxygen levels toward present values. Evidence suggests that this rise in oxygen occurred just prior to the advent of the first complex animals in the Ediacaran.
PP41A-0577 0800h
Large-Scale Neoproterozoic Slope Instability due to Lack of Bioturbation ? A Case From the Yangtze Platform (Hunan Province, Central China)
Neoproterozoic and basal Cambrian strata of the Yangtze platform, China, include fossiliferous, shallow-water, stratigraphically discontinuous carbonate environments and mostly fossil-free deep-water but stratigraphically continuous siliciclastic environments. Stratigraphic sections in the slope facies of the Neoproterozoic Doushantuo Formation, mostly in southern Hunan Province, relate these two environments to each other and constrain slope geometry and processes in the time interval following the end of the Neoproterozoic glaciation. In the sections, numerous discontinuities due to slides, olistostromes, and slump folds modify the original anoxic, shaly, cherty, and phosphatic sediments. Volumetrically most significant are 10 - 60m thick sections of gently-deformed, shallow-water dolomitized grain- and packstone slide blocks which are embedded in and overlie deep-water shales with discordant, planar surfaces displaying drag features. Correlation of these contrasting lithologies and of their contacts over tens of km suggests the existence of several regional sedimentary detachment horizons on the Hunan slope. Reconstructions constrain the slides to exceeding 70 km in lateral extent and approximately 50 km in runout distance. Stratigraphically upsection, large slump folds and slide masses continue in the Liuchapo Formation and approximately to the pC-C contact. The low degree of internal deformation in the large slide sheets may be partially caused by early-cemented, brittle phosphorites acting as basal glide planes. These phosphorites contain evaporite-replacement crystals and textures; they are likely correlatable with phosphorites that form three regional marker units on the shallow-water platform and suggest the existence of restricted, shallow-water environments to the shelf edge, abutted by a steep upper slope susceptible to regional collapse. The lack of bioturbation in the Neoproterozoic and the resulting high saturation of fine-grained sediment with organic matter, fluid, and gas, along with a strongly developed primary anisotropy from suspension settling and chemical precipitation may have facilitated the initiation of sediment displacement and increased the runout distance. Concurrently developing Doushantuo-age shelf grabens in nearby Hubei Province may favor active tectonics and associated seismicity as a possible trigger mechanism over sea-level changes.
PP41A-0578 0800h
A Priori Models to Examine the Potential for Geochronology to Determine Sedimentary Cycle Durations
Sedimentary cycles are identified in numerous depositional environments. Often they can be credibly lined to Milankovitch orbital parameters and so they provide a high resolution measuring stick for geologic processes. However few cyclic sections are dated and even those that have been dated often suffer from few and sometimes controversial ages. We generate Monte Carlo models to test the potential accuracy of cycle durations inferred from age estimates. In particular we are interested in determining the resolving capability of more, but less precise, age estimates obtained from dating syn-sedimentary carbonates, as opposed to fewer, but more precise ash ages. Our simulations consider the potential cycle duration, the number of cycles represented in a section, the precision of the ages (assumed to represent accuracy), and the number of available ages. To create the models we assumed that ashes and carbonates that can be dated are randomly distributed in the section. We use cycles as a means of partitioning time into quasi-equal increments and examine cycles that have an average duration of 20, 100, and 400 ky, consistent with well studied Milankovitch cycles. For experiments where only two high precision ages were used, we imposed the constraint that the ages had to be far enough apart to be resolvable. Such a constraint was not placed on the models where ten ages were used. Based on these parameters we ran Monte Carlo simulations to examine the potential of randomly distributed age determinations within a section with a given number of cycles to define cycle duration within that section (a least-squares fit of age determinations versus cycle position). A total of 100,000 experiments were performed for each hypothetical section to estimate the variance of determined cycle duration from the actual cycle duration. Assuming that precision does equal accuracy, a priori models demonstrate that a greater number of lower precision ages have the potential to produce a higher precision estimate of position in a section (here calculated in terms of uncertainty in cycle duration) than the case of two higher precision ages. The longer the time interval considered, the better the ability to resolve cycle duration, whatever the tool. Ten precise ash ages always perform the best but, for sections greater than 2 My in duration, ten less precise carbonates are always preferable to the two precise ash age determinations in their ability to define cycle duration.
PP41A-0579 0800h
Lower Mississippian Positive Carbon Isotope Excursion in Shallow Water Carbonates, Wyoming and Montana
Lower Mississippian carbonates from Wyoming and Montana show a positive carbon isotope excursion (maximum \delta$^{13}$C values range from +5 to +7.5 \permil PDB) within North American-Lower Mississippian/Kinderhookian to Osagean (Tournasian to Lower Visean) intervals, indicating a fluctuation in the global carbon cycle at that time. This carbon isotope excursion is found along the entire Madison ramp in cores (Elk and Bighorn Basins) and in measured sections in Wyoming and Montana. The excursion is independent of facies and occurrs in pervasively dolomitized up dip locations as well as basinward locations composed entirely of limestone. Positive carbon values are intimately associated with sequence stratigraphy previously determined in the measured sections. The onset of the transgression in Sequence II coincides with the most depleted carbon values (ca. 1 \permil PDB) and a progressive increase of enrichment during the transgression. It reaches a maximum enrichment (ca. 7 \permil PDB) at the turnaround to the regression, followed by a gradual trend from maximum values at the turnaround to minimum values (ca. 2 \permil PDB) at the top of the sequence. On the Madison ramp, the positive carbon excursion can be used to correlate third order sequence boundaries across the 1100 km ramp system, providing a time line within otherwise undatable sections. On a larger scheme, these enriched values also correlate to time-equivalent strata discovered in previous studies from Utah, Nevada, Idaho, Wyoming, Iowa, and Western Europe, indicating its potential use as a global chronostratigraphic tool. The timing of this event probably coincides with low atmospheric CO$_{2}$ levels and the initiation of conditions that lead to the Icehouse Earth in the Upper Mississippian. In order to confirm this interpretation we are measuring the \delta$^{13}$C of coexisting organic material to compare the difference between \delta$^{13}$C of the inorganic and organic components and assess pCO$_{2}$ levels and associated changes within the marine organic and inorganic carbon pools.
PP41A-0580 0800h
Preliminary $\delta$$^{13}$C and $^{87}$Sr/$^{86}$Sr Curves for Late Mississippian (Chesterian) Carbonates and Siliciclastics in East-Central Idaho
Upper Mississippian (Chesterian) carbonate and siliciclastic rocks across east-central Idaho and southwest Montana were measured and sampled for biostratigraphy and chemostratigraphy to derive the regional sequence stratigraphy. The resulting $\delta$$^{13}$C profile displays high frequency perturbations that, in conjunction with a well defined $^{87}$Sr/$^{86}$Sr curve, may provide insight into the relationship between Chesterian seawater chemistry and relative sea level history. A collection interval of 0.5 meters was applied to a $>$500 meter composite section from Idaho, and whole rock analysis of the 700 resulting samples was augmented by component analysis of brachiopods and marine cements. Sample staining and cathodoluminescence of select thin sections attest to the integrity of the data set. The average maximum and minimum values for the resulting $\delta$$^{13}$C curve are +3.0$\permil$ (PDB) and -3.0$\permil$ (PDB), respectively. A number of long term trends are evident in the curve. Except for an initial abrupt decline from +2$\permil$ to -2$\permil$, lower Chesterian values average +1.5$\permil$ until a significant negative excursion to -1$\permil$ and -2$\permil$ in the middle Chesterian. This boundary coincides with a sea level fall, recognized stratigraphically as an abrupt transition from carbonate to calcareous siltstone. Uniform cyclic fluctuations remain between -1 and +1$\permil$ until truncated by a rise to +3$\permil$. The resulting increased amplitude persists into latest Chesterian oscillations; values fall from +3 to -2.5$\permil$ and then increase to an concluding average of +2$\permil$. Superimposed on the longer term curve are multiple smaller fluctuations that appear, in some cases, to correspond with shallowing upward depositional sequences. Overall per mil values are lower than published Chesterian averages, but trends in the curve appear similar to those in the $\delta$$^{13}$C curve established for the North American mid-continent (Mii et al, 1999). $^{87}$Sr/$^{86}$Sr values of thick shelled brachiopods were obtained by laser ablation (MC)-ICP-MS. The resulting curve delineates a rise in $^{87}$Sr/$^{86}$Sr ratios that is in accord with published trends for the Carboniferous, and that may be used in establishing a chronologic framework for the observed $\delta$$^{13}$C signature.
PP41A-0581 0800h
Perturbations to the Carbon and Sulfur Cycle During the Permian-Triassic Boundary Event in Southern China
The Permian-Triassic boundary (251 Ma) is a time of major change in the chemistry of the oceans, as well as the period of the greatest mass extinction of the Phanerozoic. The cause of these changes remains a subject of intense research. Many of the current theories rely on changes to the oceanic sulfur cycle. Carbonate associated sulfate has the potential to provide a high resolution record of variations in the concentration and stable isotopic composition of seawater sulfate. This is of interest because the majority of the current data in this area is from evaporites or pyrites both of which are relatively rare in the stratigraphic record. The isotopic value of seawater sulfate is known to shift from approximately $13\permil$ in the late Permian to $35\permil$ during the early Triassic. Our research focuses on CAS found in marine carbonates, and we have obtained samples from two sections that span the Permian-Triassic boundary (PTB) of the Meishan and Changxi sections located in Southern China. The carbonates were powdered and the CAS was extracted as barite. Sulfate content peaks at the event horizon at Meishan, indicating perhaps either substantial changes in local (or global) surface ocean chemistry, pH changes, or changes in rates of mineral precipitation. Isotopic analysis of the CAS provides a detailed record of the isotopic shift is $\delta$$^{34}$S across the PTB in two temporally constrained sections, and $\delta$$^{18}$O of sulfate also aids in our evaluation of the timing and extent of ocean anoxic events. The $\delta$$^{13}$C of kerogen at Meishan reaches a minimum just before the event horizon and the point of lowest $\delta$$^{13}$C for carbonate. This differs from many other PTB sections where both sets of carbon isotopes reach a minimum simultaneously. This is not as clear in the Changxi section where the event horizon is not as well resolved. The possibility of primary or secondary diagenetic alteration is also being examined for both sections. The combination of these two data sets provides a concise temporal record of changes in the oceanic sulfur cycle, as well as the changes in the carbon cycle across the PTB.
PP41A-0582 0800h
Causes of the marine productivity and oxygen changes associated with the Permian - Triassic boundary: A reevaluation with general ocean circulation models
Widespread anoxic and dysoxic deposition in marine sediments has been associated with a stagnant ocean at the Permian-Triassic (P-Tr) boundary. This P-Tr boundary is also associated with one of the largest mass extinctions of species in the Paleozoic. Here we review recent three-dimensional numerical model studies that investigated possible circulation patterns at the P-Tr boundary. In addition, we conduct new sensitivity experiments with a coupled ocean-atmosphere model of intermediate complexity including marine chemistry to study changes of the deep sea oxygen distribution associated with 1) a significant change in the hydrological cycle and 2) a massive release of methane gas hydrates from marine sediments. Freshening of sea surface water masses in the magnitude of the Permian deglaciation produces a significant local minimum in oxygen in the eastern tropical Panthalassa ocean at intermediate depth and at the sea floor. The methane release experiment simulates a strong increase in sea surface temperature (about 7 K) and a vigorous deep sea circulation. In contrast to the conclusions drawn from previous studies, this reanalysis of the ocean circulation at the P-Tr boundary favors a strong ventilation in most parts of the deep sea. Thus, a significant change in the oceanic carbon inventories and a reorganization of the marine productivity associated with a stronger biological pump may be required to explain the anoxic sedimentary deposits.
http://drake.aos.wisc.edu/~winguth/Papers/Papers1.html
PP41A-0583 0800h
Causes and Effects of the early Aptian ($\sim$117 Ma) Methane Release
In 2001, we reported a negative excursion in early-Aptian atmospheric $\delta^{13}$CO$_2$ ($\Delta$ = -3.6 to -6.5 $\permil$), based on $\delta^{13}$C analyses of organic matter and land-plant isolates from coarsely-sampled Colombian estuarine and near-shore sediments. Here we present similar results for an Aptian section of the Arundel Clay (Potomac Group, central Maryland), which is well-known for its exceptional preservation of terrestrial plant materials. Sampling across 13 meters of sediment at $\sim$10-cm intervals revealed a clear shift in the $\delta^{13}$C of terrestrial organic matter ($\it n$=153) and land-plant isolates ($\it n$=33) of $\Delta$ = -2.3 and -2.9 $\permil$, respectively. The shift was observed within palynological Zone I, which is temporally well-correlated with our previous work. Given the probable composition of the early Cretaceous atmosphere, a methane hydrate release is the likely cause of this excursion; isotopic mass balance of our record in conjunction with the $\delta^{13}$C$_{carbonate}$ record of Menegatti et al., 1998 suggest a total methane hydrate C release =$\sim$ 1,100 Gt ($\sim$10$%$ of the modern reservoir) over a period of approximately 500 kyr. In consideration of a mechanism for early Aptian methane release, we calculated changes in global subduction rates during the Early Cretaceous from the classic high-resolution plate reconstructions established by Engebretson,1985. These reconstructions revealed a dramatic decrease in the motion of the Farallon plate toward the subduction zones of the North Pacific basin during the early Aptian, caused by a massive increase in frictional interaction (i.e., seismic coupling) between overriding and subducting plates stretching from northeast Asia, to Alaska, to British Columbia. Associated forces caused uplift and compression in continental margins sufficient to continuously destabilize a portion of the probable methane hydrate reservoir (evidence of this compression is also observed in the geologic record [Vaughan et al. 1995]). The methane hydrate release created a perturbation in early Aptian terrestrial and marine C reservoirs, as reflected in the $\delta^{13}$C of terrestrial and marine organisms. We speculate further as to the influence of this methane release on the Early Cretaceous atmosphere as well as early Aptian OAE events. Marine oxidation of the CH$_4$ implied by the marine carbonate $\delta^{13}$C record would consume =$\sim$ 5,500 Gt of O$_2$, enough to remove at least 60$%$ of the O$_2$ dissolved in the below-surface ocean, perhaps leading to anoxic conditions at depth. Furthermore, terrestrial oxidation of released CH$_4$ to CO$_2$ would increase the atmospheric ratio of CO$_2$:O$_2$ from 0.007 to 0.008, perhaps facilitating the geographic expansion of the angiosperms (which have been hypothesized to out-compete gymnosperms near values of 0.01 [Kienast, 1991]). Over and above these speculations, the Aptian methane release is a new example of mechanistic coupling between major tectonic events and the global biosphere, as recorded in the stable isotope record.
PP41A-0584 0800h
Stable Isotopic Records of the Terrestrial Paleoclimatic Consequences of mid-Cretaceous (Aptian-Albian) Oceanic Anoxic Events
Linkage between Aptian-Albian marine, atmospheric, and terrestrial carbon reservoirs is shown by the d13C chemostratigraphy of pedogenic carbonates in the Cretaceous proximal foreland basin fill of eastern Utah. Profiles from sections that accumulated in the foredeep and forebulge have a well-organized structure that we correlate to Aptian positive d13C excursions associated with Oceanic Anoxic Events (OAEs) 1a and 1b, and support the use of pedogenic carbonates for continental-marine chemostratigraphic correlations. The 5 % shifts in our continental sections exceed magnitudes reported from coeval marine carbonate sections, duplicating published results on Aptian d13C changes in terrestrial C3 plants. Peak d13C values of -3 % in the foredeep section occur in Aptian excursions associated with OAEs 1a and 1b, whereas minimum d13C values of -9.3 % occur in the forebulge section in Albian strata. A Late Albian positive excursion of 2 % in the foredeep section is correlated with the 400 Kyr OAE 1d (Wilson and Norris, 2001, Nature 412:525-429), underscoring the potential for high-resolution terrestrial correlation of Cretaceous OAEs. Pedogenic carbonates in background positions of the d13C profiles contain meteoric phreatic components that yield meteoric calcite line trends with d18O values of about -8 %. This contrasts with carbonate d18O values of about -3 % at the position of OAE 1a (segment C7 of Bralower et al., 1999, Jour. Foram. Res. 29:418-437), of about -5 % at the position of OAE 1b (segments C9-C11), and of about -4 % at the position of OAE 1d. Carbonates at the positions of these positive d13C excursions include partially dolomitized rocks composed of interlocking 50-150 micron-sized dolomite rhombs overprinting relict syntaxial inclusions of poikilotopic calcite spars. The calcite spars and overprinting dolomites have isotopic values that are arrayed along positive linear covariant trends in C & O isotope space. We interpret these strata to record the sabkha-type dolomitization of calcretes by hyperconcentrated pore fluids, resulting from local aridification of terrestrial paleoenvironments in the proximal foreland basin during Aptian-Albian OAEs.
PP41A-0585 0800h
The Toarcian Oceanic Anoxic Event (T-OAE): Global v Local Causes
The rapid and massive dissociation of methane from gas-hydrates has been proposed as a trigger for the Toarcian Oceanic Anoxic Event (T-OAE) and potentially could explain simultaneous large negative excursions (up to 7$\permil$ PDB) in bulk carbonate ($\delta$$^{13}$C$_{carb}$) and organic carbon isotope records ($\delta$$^{13}$C$_{org}$) from across sections in NW Europe. After oxidation of the released methane, raised atmospheric carbon dioxide levels are thought to have caused global greenhouse climate conditions. Acceleration of the hydrological cycle and persistently high rates of weathering and run-off fuelled increased primary production, which aided by salinity stratification, led to the deposition of widespread black shales with TOC up to 18%. We present C-isotope records from belemnites ($\delta$$^{13}$C$_{bel}$) sampled from two localities, calibrated with high-resolution ammonite biostratigraphy, in Yorkshire (England) and Dotternhausen (Germany), that do not show such rapid and large negative C-isotope excursions when compared with coeval ($\delta$$^{13}$C$_{org}$) and ($\delta$$^{13}$C$_{carb}$) records. Therefore light carbon can not have dominated the entire carbon system, as would be expected in the case of methane dissociation. Based on a critical evaluation of all carbon isotope records, including our ($\delta$$^{13}$C$_{bel}$), we support a model in which the recycling of DIC from the deeper levels of a stratified water body and shallowing of anoxic conditions into the photic zone play a central role. A compilation and comparison of ($\Delta$$\delta$$^{13}$C$_{carb-org}$) records from sections across an inshore-offshore transect further illustrates that belemnites recorded a more open ocean (positive) carbon isotope signal, while (bulk) carbonate and organic carbon tracked the degree of stratification and related carbon dioxide pressure.
PP41A-0586 0800h
Neogene Carbonate Platforms as Recorders of the Onset and Strengthening of the East Asian Monsoon
Miocene to Recent carbonate platforms from southern parts of the South China Sea and Indonesian back-arc region have variable internal growth patterns that likely reflect the onset and strengthening of the East Asian Monsoon, an annual seasonal cycle where precipitation, and more importantly for this study, wind directions change dramatically. Extensive high-quality seismic-reflection data were examined across the study area and showed that isolated carbonate platforms commonly have asymmetrical facies distributions and internal stratal patterns that reflect prevailing paleowind directions during growth. Early to middle Miocene platforms throughout the study area show consistent windward-leeward asymmetry, which suggests that prevailing winds were fairly constant over long time scales. Starting at about 11 Ma, however, growth patterns became much more variable in many platforms, especially those from the East Natuna Basin and offshore Vietnam. These complex growth patterns continue until the present day. In the Indonesian backarc region, middle to late Miocene isolated platforms show strongly asymmetrical but consistent growth patterns that also reflect strengthening of the East Asian Monsoon at this time because prevailing winds during the winter monsoon are out of the west and stronger than the weaker, easterly winds of the summer monsoon. Thus, Neogene isolated carbonate platforms of the southern South China Sea and Indonesian backarc region may be some of the best recorders of the evolution of the East Asian Monsoon.
PP41A-0587 0800h
Rare Earth Element Evidence for Paleolake Chemistry
The Eocene Green River Formation contains lacustrine deposits famous for their oil shales, fish fossils and cyclic nature. Although there are common trends in the facies and types of authigenic minerals seen in progressive lake phases and among the different lake basins, there are also distinct differences. These differences illustrate that the basins were not connected through at least part of their history. The common trends throughout the sequences and from all the basins include: 1) elevated alkalinity; 2) cycles that reflect contraction (elevated salinity) and expansion (freshening); and 3) sedimentary structures that allow us to place facies in a limno-geologic context. A key goal is to relate the facies to the paleo-lake chemistry. This requires comparison to modern lakes where similar facies are being created coupled with an integration of the chemistry of the fluids and the trace element patterns in the authigenic minerals. REE patterns from the Great Basin Lake waters are HREE enriched and show trends of increased REE concentrations that have been linked directly to alkalinity (Johannesson et al., 1994). Authigenic phases from facies that reflect lowstands, transgression and highstands in the Green River Formation have patterns that can be directly compared to those from the Great Basin Lake waters. This illustrates great potential for constraining the alkalinity of the precipitating fluids. Although the Kd of the authigenic minerals for the REE's has to be considered before conclusive models can be proposed, there are impressive similarities between REE patterns of authigenic phases from the Green River Formation and waters of modern alkaline lakes of the Great Basin.
PP41A-0588 0800h
Late Cenozoic reduction in Antarctic Circumpolar Current flow from analyses of drift deposits along the Antarctic Peninsula, ODP Site 1095.
All waters from the Earth's oceans flow into and are mixed in the Antarctic Circumpolar Current (ACC). Thus the ACC has a major effect on the transport of heat in the oceans and the Earth's climate. Sediment drifts, which form along the Antarctic Peninsula under the influence of the ACC, record the history of this flow. We examined samples from cores drilled during Ocean Drilling Program (ODP) Leg 178 along the Antarctic Peninsula to determine paleocurrent strength changes. Both paleomagnetic cubes and sediment samples were taken from cores from ODP Site 1095 ($66\deg$59'S, $78\deg$29'W, 3814m). Factor analysis using major element chemistry indicates a two-component system between terrigenous and biogenic opal. The terrigenous fraction of 236 sediment samples was isolated and the grain size measured to determine the terrigenous mass accumulation rate (MAR), the median grain size ($\phi$50), the $>$63 $\mu$m MAR and the biogenic opal MAR. Terrigenous mass accumulation rates reflect the overall MAR's at the site, decreasing in a series of steps at 9 my, 5.5 my, and 2.5 my. Median grain size changes occur at 5.5 my (sharp decrease followed by a slight increase) and 2.7 my (decrease). The magnetic fabric, as determined for 324 samples by anisotropy of magnetic susceptibility (AMS), decreases sharply at 5.7 my and again at 2.7 my. We interpret these step-like changes as indicating a decline in the current strength over the last 6 my, with specific decreases occurring around 5.5 my and 2.5 my.