John M. Jaeger

Sediment deposition, accumulation, and seabed dynamics in an energetic fine-grained coastal environment

Sedimentary processes on the continental shelf and shoreline northwest of the Amazon River mouth were investigated as part of A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds) during four field expeditions between 1989 and 1991. Periodic deposition and resuspension of seabed layers as much as a meter thick dominate sedimentary processes for most of the inner shelf and for the shoreface and foreshore north of Cabo Cassipore. Strata forming as a result of this process consist of decimeter-thick mud beds separated by hiatal (scour) surfaces. The volume of sediment resuspended seasonally from the inner shelf surface layer (SL) is of the same order of magnitude as the annual input from the river, indicating that resuspension is an important control on suspended-sediment distributions in shelf waters. Most resuspension from the SL occurs during February–May (the period of maximum wind stress), which is also the time of rapid deposition on the mudflats, suggesting that sediment resuspended from the SL could contribute to shoreface and foreshore accretion for the northern portion of the study area. In addition, some of the sediment resuspended from the SL is transported seaward periodically in the form of near-bottom fluid-mud flows. This results in non-steady-state input of certain particle-reactive trace metals, which is reflected in the occurrence of quasi-cyclic210Ph profiles in the foreset region of the subaqueous delta. As determined using228Ra/226Ra geochronology, sediment accumulation rates in this region are 10–60 cm y−1. Farther seaward, in the bottomset region, accumulation rates decrease and there is increased evidence of biological activity preserved in sedimentary structures. However, episodic (but reduced) sediment input from fluid-mud flows also extends to this region, affecting the fauna and fine-scale stratigraphy.

Tidal controls on the formation of fine-scale sedimentary strata near the Amazon river mouth

Abstract The Amazon river mouth provides a dynamic setting for studying the formation of sedimentary strata under conditions where fluvial and marine processes merge. River-mouth anchor stations were occupied for diurnal tidal cycles during three stages of river flow, and reoccupied for consecutive spring and neap tides during two stages of river flow. At each anchor station, box cores were collected every two hours and complementary time-series measurements were made of water-column suspended-sediment concentrations, salinity, and current velocities. During high-energy periods in the fortnightly cycle (i.e. most spring tides), a cross-laminated sand layer is present at the seabed surface that exhibits varying degrees of bi-directional current structure and contains low porewater salinities (10–15‰). During low-energy periods in the fortnightly cycle (i.e., most neap tides), a mud bed forms at the seabed surface in association with fluid muds in the water column. This mud layer ranges in thickness from 2 to 15 cm, and exhibits higher radionuclide activities, higher porewater salinities (15–25‰), and lower saturated bulk densities (1.18–1.30 g/cm3) than the sand beds (1.30–1.60 g/cm3). The mud beds are subject to resuspension and deposition by semidiurnal tidal currents that form thin sandy interlaminations. Interlamination and interbedding of sand and mud result from the combination of estuarine and tidal processes at the river mouth. Interlaminations (alternating layers of sand and mud 1 cm in thickness) muds and sands. The thickness of the fortnightly beds is dependent upon monthly variations in spring/neap amplitudes. The processes active near the Amazon river mouth that form interlaminated and interbedded sediments operate in other fluvial-marine settings, and produce similar types of interlayered sediments due to the presence of estuarine circulation, high suspended-sediment concentrations, and tidal energy.

Lateral transport of settling particles in the Ross Sea and implications for the fate of biogenic material

The Ross Sea, Antarctica, with its high rates of primary productivity and biogenic accumulation, provides an important location to test the validity of a one-dimensional particle-settling model. As part of an interdisciplinary field project performed from 1990 to 1992 to examine cycling and accumulation of biogenic matter in the Ross Sea, water-column particulate and current data were collected at three sites. At each of the sites, a current meter and sediment trap were placed 240 m below the water surface, and a similar set of instruments was located 40 m above the seabed. The moorings were deployed for 1- to 2-years duration. The current-meter records showed that the speed of flow in the southwestern Ross Sea is relatively slow ( 50 cm s−1) and least variability in direction. To examine the validity of a one-dimensional approximation for fluxes of biogenic material, two models were developed to determine the net displacement of particles settling through the water column. Current-meter data and particle-settling characteristics were incorporated in both models. One model produced a time-varying, linearly interpolated current field between the moorings in which particle advection was evaluated. The second model used time-averaged progressive-vector plots to estimate lateral particle advection. Results show that particles are displaced the least at the southwestern site ( 50 km). The pattern in displacement trends correlates well with observed sediment types and accumulation rates at each site. A one-dimensional model for the settling of biogenic material is most applicable at the southwestern site and least applicable at the northwestern site.

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska.

Significance In coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This finding underscores the power of climate in driving erosion rates, potential feedback mechanisms linking climate, erosion, and tectonics, and the complex nature of climate−tectonic coupling in transient responses toward longer-term dynamic equilibration of landscapes with ever-changing environments. Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Neotectonics of the Yakutat Collision: Changes in Deformation Driven by Mass Redistribution

The most recent period of orogenesis in southern Alaska began in the late Neogene with the collision of the Yakutat microplate, which is partially accreted to and partially subducted beneath the Alaskan margin at the easternmost extent of the Aleutian Trench. Neotectonic studies suggest significant spatial and kinematic variation in active deformation during the collision of the Yakutat microplate. The Saint Elias orogen experienced a widespread structural reorganization in the Quaternary with oblique convergence partitioned onto an en echelon thrust array. The new tectonic configuration also includes the continuing development of an incipient indentor comer, significant retrothrust motion, and shifting deformation fronts. Reorganization is temporally linked to intense glacial erosion in the core of orogen and rapid sedimentation in offshore depocenters during the Pleistocene. We propose that mass redistribution and modification of orogenic topography played an integral role in the structural and tectonic evolution of the present system. Currently, the spatial deformation front (outboard limit of deformation) and active deformation front are not the same, suggesting that deformation swept through the landscape through time, presumably as a result of glaciation, tectonic adjustment, or both. A more complete picture of the complex response of near-surface deformation to topographic disruption should improve seismic hazard assessments.

Fjords as temporary sediment traps: History of glacial erosion and deposition in Muir Inlet, Glacier Bay National Park, southeastern Alaska

Glacimarine sedimentary deposits within the basins of Muir Inlet, a 48-km-long silled fjord, are interpreted from complimentary sets of high-resolution, seismic-reflection profiles using known glacial-advance and retreat history. Two prominent glacial erosion surfaces are identified: the lowest attributed to the Last Glacial Maximum (LGM) advance and the upper coincident with the Little Ice Age (LIA) advance. The LGM ice sheet, which advanced onto the continental shelf, was 1700 m thick in Muir Inlet and eroded bedrock, whereas the thinner LIA ice did not. LGM deposits >300 m thick occur beneath the LIA erosion surface in the deepest basins. Evidence for earlier Neoglacial advances is present in subaerial deposits; however, Neoglacial sediments preserved within the marine record are restricted to one depositional package on the entrance sill. Volumes of LIA retreat sediments were calculated within basins. An average annual sediment flux was calculated by modeling the duration of sediment contributed from Muir Glacier and from tributary glaciers and side-entry sources. The annual sediment flux ranged from 1.3 × 10 6 m 3 /yr to 4.6 × 10 7 m 3 /yr and increases logarithmically with increasing drainage basin area, similar to fluvial systems. This sediment flux does not only represent bedrock erosion. Additional sediment is contributed from persistent tributary glaciers and from LGM sediment stored within deeper basins. Basin-wide reflections characterize the most common seismic facies and indicate that strata are horizontal and continuous across each basin, confirming the importance of sediment gravity flows originating from sills and sloping fjord walls.

Implications of carbon flux from the Cascadia accretionary prism: results from long-term, in situ measurements at ODP Site 892B

A 403-day in situ field experiment at Ocean Drilling Program Site 892B sought to quantify the flux of methane along a fluid-active fault and to experimentally determine rates of methane hydrate and authigenic carbonate deposition associated with fluid expulsion from the borehole. An instrument package was deployed that osmotically sampled fluid, measured borehole pressure and flow rates, and contained reaction chambers in which deposition of gas hydrates and carbonates was anticipated, and from which microbial communities might be extracted. Flow is highly variable in the three-phase water–methane system that exists at Site 892B. Flow rates fluctuate over two orders of magnitude in response to tidally induced pressure variations and gas hydrate formation and dissociation. Hydrate formation began 45 days into the experiment and reduced the initial flow (∼2 l/day) to 20 ml/day. Unexpectedly, the hydrate destabilized after about 125 days. Tidally induced flow reversals are common (∼25% of time) in this setting characterized by ‘overpressured’ pore waters. These reversals pump sulfate-rich bottom water into near-surface sediments where Archaea anaerobically oxidize CH4 and induce carbonate precipitation. At the sediment–water interface, authigenic carbonates are undergoing dissolution. Methanotrophs dominated the microbial community where fluid is discharged to ambient seawater. All expelled methane is apparently oxidized in the water column.

MARINE RECORD OF SURGE-INDUCED OUTBURST FLOODS FROM THE BERING GLACIER, ALASKA

The Bering Glacier, Alaska, is the largest temperate glacier in the world. It episodically surges with rapid advances of the glacier terminus followed by large outburst floods delivering freshwater and sediment to the adjacent Gulf of Alaska. We describe the marine record of the 1993–1995 surge and document a 100 yr history of surges recorded in marine sedimentary deposits seaward of the Bering Glacier. In 1994 and 1995, we collected box cores that contained high-porosity laminated sediments at the seabed surface. Profiles of 234 Th and chlorophyll-a indicate that these sediments were deposited very rapidly (0.1 cmṁday −1 ) in association with the surge. A 250-cm-long kasten core extended this record, in which 7 laminated beds, 10–30 cm thick, alternated with bioturbated sediments. On the basis of 210 Pb chronology, 6 of these beds accumulated in the past 100 yr and can be correlated with historical surges.

Isostatic uplift driven by karstification and sea-level oscillation: Modeling landscape evolution in north Florida

Isostatic uplift of tectonically stable, passive margin lithosphere can preserve a record of paleo-shoreline position by elevating coastal geomorphic features above the influence of nearshore wave activity. Conversely, depositional ages and modern elevations of these features can provide valuable information about the uplift history of a region. We present a numerical model that combines sea-level oscillation, subaerial exposure, a precipitation-karstification function, and isostatic uplift to explore the dynamic geomorphic behavior of coastal carbonate landscapes over multiple sea-level cycles. The model is used to estimate ages of coastal highstand depositional features along the Atlantic coast of north Florida. Numerical simulations using current best estimates for Pleistocene sea-level and precipitation histories suggest ages for Trail Ridge (1.44 Ma), the Penholoway Terrace (408 ka), and the Talbot terrace (120 ka) that are in agreement with fossil evidence. In addition, model results indicate that the rate of karstification (void space creation or equivalent surface lowering rate) within the north Florida platform is ∼3.5 times that of previous estimates (1 m/11.2 k.y. vs. 1 m/38 k.y.), and uplift rate is ∼2 times as high as previously thought (0.047 mm/yr vs. 0.024 mm/yr). This process has implications for landscape evolution in other carbonate settings and may play an underappreciated role within the global carbon cycle.

Orogenic and glacial research in pristine southern Alaska

Southern Alaska is an exceptional natural laboratory for studying a range of geologic problems, including the links between orogenic processes, landscape modification by glacial processes, and continental margin sedimentation. Geologic processes operate at rapid rates along the southern Alaskan margin, which allows scientists to concurrently collect data on tectonic deformation, uplift, erosion, and sedimentation, and develop comprehensive models that connect these diverse processes. Significant advancements have been made in studying fundamental geologic processes in this region. However, efforts to link these into comprehensive models are in their infancy and fundamental research questions remain. The National Science Foundations MARGINS Source-to-Sink program (see http://www.ldeo.columbia.edu/margins/SciencePlan.15Novpdf) will provide an interdisciplinary group of geoscientists the opportunity to merge these studies and significantly advance our understanding of continental margins.