Glacimarine sedimentary deposits within the basins of Muir Inlet, a 48-km-long silled fjord, are interpreted from complimentary sets of high-resolution, seismic-refl ection profi les using known glacial-advance and retreat history. Two prominent glacial erosion surfaces are identifi ed: 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 Neo glacial 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 fl ux was calculated by modeling the duration of sediment contributed from Muir Glacier and from tributary glaciers and side-entry sources. The annual sediment fl ux 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 fl uvial systems. This sediment fl ux does not only represent bedrock erosion. Additional sediment is contributed from persistent tributary glaciers and from LGM sediment stored within deeper basins. Basin-wide refl ections characterize the most common seismic facies and indicate that strata are horizontal and continuous across each basin, confi rming the importance of sediment gravity fl ows originating from sills and sloping fjord walls.
Glacimarine sediment deposited in the fjord adjacent to Muir Glacier in south-eastern Alaska consists of rhythmically laminated muds, strati®ed sandy mud, sand and gravelly mud facies. Cyclicity is recorded by gravelly mud facies deposited during winter by ice-rafting, black mud laminae formed by spring plankton blooms and variations in tidal rhythmite thickness and texture produced by the interaction of meltwater discharges and tidal currents in the macrotidal fjord. Regular cyclicity in laminae thickness is tested statistically by Fourier transform and can be attributed to a lunar tidal cycle control in the ®ve cores collected up to 6 km from the sediment source. Cores close to the source can have additional laminae as a result of dischargē uctuations, and distal cores may lack full cycles because of variability in the plume path and attenuation with distance. Cyclic variations in sediment texture are recorded in magnetic susceptibility (MS) pro®les of the cores.High MS values are produced by turbidite sand beds or by strati®ed sandy mud deposited by over¯ow plumes during peak summer meltwater discharge. Low values re¯ect muddy intervals deposited during periods of low meltwater discharge, such as during autumn and winter. Sediment accumulation rates measured by 210 Pb dating range from 82 cm year ±1 , 2 km from the sediment source at the head of the fjord, to 16 cm year ±1 , 6 km away. These rates are within the same range as average sediment accumulation rates determined from cyclic seasonal markers within the cores. These data show that, with careful documentation, annual cycles of glacimarine sediment accumulation can be detected within marine cores. Cores collected from the distal portion of the basin were deposited during the transition of Muir Glacier from a tidewater terminus ending in deep water to a terrestrial glacier with an ice-contact delta deposited in front of the terminus. This transition is recorded by a coarsening-upward sedimentary sequence formed by turbidite sands originating from the prograding delta above ®ne-grained, laminated basin ®ll deposited by turbid over¯ow plumes.
Elevated
concentrations of toxic elements in coal ash pose human
and ecological health risks upon release to the environment. Despite
wide public concerns about water quality and human health risks from
catastrophic coal ash spills and chronic leaking of coal ash ponds,
coal ash disposal has only been partially regulated, and its impacts
on aquatic sediment quality and ecological health have been overlooked.
Here, we present a multiproxy approach of morphologic, magnetic, geochemical,
and Sr isotopic analyses, revealing unmonitored coal ash releases
over the past 40 to 70 years preserved in the sediment records of
five freshwater lakes adjacent to coal-fired power plants across North
Carolina. We detected significant sediment contamination and potential
chronic ecological risks posed by the occurrence of hundreds of thousands
of tons of coal ash solids mainly resulting from high-magnitude stormwater
runoff/flooding and direct effluent discharge from coal ash disposal
sites. The proximity of hundreds of disposal sites to natural waterways
across the U.S. implies that such contamination is likely prevalent
nationwide and expected to worsen with climate change.
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