Date List X contains an annotated listing of 213 radiocarbon dates determined on samples from marine and terrestrial environments. The marine samples were collected from the East Greenland, Iceland, Spitzbergen, and Norwegian margins, Baffin Bay, and Labrador Sea. The terrestrial samples were collected from Vestfirdir, Iceland and Baffin Island. The samples were submitted by INSTAAR and researchers affiliated with INSTAAR's Micropaleontology Laboratory under the direction of Dr.’s John T. Andrews and Anne E. Jennings. All of the dates from marine sediment cores were determined from either shells or foraminifera (both benthic and planktic). All dates were obtained by the Accelerator Mass Spectrometry (AMS) method. Regions of concentrated marine research include: Baffin Bay, Baffin Island, Labrador Sea, East Greenland fjords, shelf and slope, Denmark Strait, the southwestern and northwestern Iceland shelves, and Vestfirdir, Iceland. The non-marine radiocarbon dates are from peat, wood, plant microfossils, and mollusc. The radiocarbon dates have been used to address a variety of research objectives such as: 1. determining the timing of northern hemisphere high latitude environmental changes including glacier advance and retreat, and 2. assessing the accuracy of a fluctuating reservoir correction. Thus, most of the dates constrain the timing, rate, and interaction of late Quaternary paleoenvironmental fluctuations in sea level, glacier extent, sediment input, and changes in ocean circulation patterns. Where significant, stratigraphic and sample contexts are presented for each core to document the basis for interpretations.
Ground-penetrating-radar (GPR) profiles calibrated with core data allow accurate assessments of coastal barrier volumes. We applied this procedure successfully to the barrier system along Saco Bay, Maine (USA), as part of a sediment-budget study that focused on present-day sand volumes in various coastal, shoreface, and inner-shelf lithosomes, and on sand fluxes that have affected the volume or distribution of sand in these sediment bodies through time. On GPR profiles, the components of the barrier lithosome are readily differentiated from other facies, except where the radar signal is attenuated by brackish or salty groundwater. Significant differences between dielectric properties of the barrier lithosome and other units commonly result in strong boundary reflectors. The mostly sandy barrier sediments allow deep penetration of GPR waves, in contrast to finer-grained strata and till-covered bedrock. Within the Saco Bay barrier system, of sediment are unevenly distributed. Two-thirds of the total barrier volume is contained within the northern and southern ends of the study area, in the Pine Point spit and the Ferry Beach/Goosefare complex, respectively. The central area around Old Orchard Beach is locally covered by only a thin veneer of barrier sand, averaging >3 m, that unconformably overlies shallow pre-Holocene facies. The prominence of barrier-spit facies and the distribution pattern of back-barrier sediments indicate that a high degree of segmentation, governed by antecedent topography, has affected the development of the Saco Bay barrier system. The present-day configuration of the barrier and back-barrier region along Saco Bay, however, conceals much of its early compartmentalized character.
Saco Bay, in southern coastal Maine, is bordered to the northwest by a sandy barrier beach framed by bedrock headlands. Although the barrier has not migrated significantly during the late Holocene, large alongshore sediment redistributions within the system have occurred in this century. Drastic coastal erosion in some local communities has spurred research aimed at quantitatively describing the Saco Bay sediment budget. Depositional products in the nearshore regions of the bay, out to about 10 km offshore, were analyzed with 178 km of seismic reflection profiles and 29 km[sup 2] of side-scan sonographs collected in 1991--1992. Vibracores confirm seismic interpretations of a Holocene shoreface wedge underlain by glaciomarine mud. The textural boundary between sand and glaciomarine mud in nearshore cores marks the Holocene ravinement (transgressive) unconformity. Throughout the 15--60 m depth zone, glaciomarine reflectors are truncated at the seafloor. Side-scan sonar interpretations of seafloor textures were confirmed by 175 existing bottom grab samples. Large areas of rippled sand and gravel occur in depths from 10--40m. These features suggest sediment transport, but rates are not easily quantified. An isopach map created using a geographic information system shows the shoreface to be the largest sand reservoir in the Saco Bay system. Themore » glaciomarine sediments in Saco Bay were deposited during deglaciation, at a time of higher than present local relative sea level. The Saco River delivered large volumes of sand of the area during the postglacial relative sea-level lowstand. This fluvial sediment supply has probably persisted, though at a decreasing rate, during the Holocene. The sandy barrier system in Saco Bay developed as sea level rose and the glaciomarine and fluvial deposits were reworked and transported.« less
Established maintenance dredging practices in ports and harbours are not always optimal, and improvements can often be made to enhance both the economic and the environmental management of a port authority. This paper describes a recent example where both economic and environmental improvements were achieved.
Kolkata Port has been in operation for over two centuries, with the Haldia Dock Complex being operational for almost 40 years. During this period, maintenance of the approach to the ports has been an ongoing concern and this region has the largest maintenance dredging burden of all Indian ports.
HR Wallingford was initially commissioned by WAPCOS Ltd (A Government of India Undertaking) in August 2012 to provide an ‘Expert Third Party Opinion on River Regulatory Measures’ on the Hugli Estuary. A review was undertaken which identified short-term improvement measures. The drivers for the subsequent project (and the focus of this paper) were to improve the efficiency of the maintenance dredging works, reduce dredging costs, and provide increased channel depth. The novel short-term solution identified successfully achieved this and allowed a drastic increase in dredging efficiency. Moreover, the change in methodology allowed significant environmental improvements to be made: carbon dioxide emissions were significantly reduced due to reduced sailing distances, and sediment was better kept within the sediment transport system with a major reduction in offshore placement. Turbidity was increased as a consequence of the short-term solution used, but this was in an environment already characterised by high natural turbidities and subject to anthropogenic changes.
This paper describes the substantial challenges addressed by the project, the novel solution that was identified, trialled and implemented by the project team (including observations from site visits), and the search for a win-win solution.
A mixing model derived from first principles describes the bulk density (BD) of intertidal wetland sediments as a function of loss on ignition (LOI). The model assumes that the bulk volume of sediment equates to the sum of self-packing volumes of organic and mineral components or BD = 1/[LOI/k1 + (1-LOI)/k2], where k1 and k2 are the self-packing densities of the pure organic and inorganic components, respectively. The model explained 78% of the variability in total BD when fitted to 5075 measurements drawn from 33 wetlands distributed around the conterminous United States. The values of k1 and k2 were estimated to be 0.085 ± 0.0007 g cm-3 and 1.99 ± 0.028 g cm-3, respectively. Based on the fitted organic density (k1) and constrained by primary production, the model suggests that the maximum steady state accretion arising from the sequestration of refractory organic matter is ≤ 0.3 cm yr-1. Thus, tidal peatlands are unlikely to indefinitely survive a higher rate of sea-level rise in the absence of a significant source of mineral sediment. Application of k2 to a mineral sediment load typical of East and eastern Gulf Coast estuaries gives a vertical accretion rate from inorganic sediment of 0.2 cm yr-1. Total steady state accretion is the sum of the parts and therefore should not be greater than 0.5 cm yr-1 under the assumptions of the model. Accretion rates could deviate from this value depending on variation in plant productivity, root:shoot ratio, suspended sediment concentration, sediment-capture efficiency, and episodic events.
[1] Hulbe et al. [2004] argue that the original binge-purge model of their coauthor MacAyeal [1993a, 1993b] is not appropriate for Heinrich events but that a new ice-shelf-collapse mechanism may work. We believe that the new collapse mechanism disagrees with important data and that MacAyeal's original model remains viable after appropriate modifications. We have enjoyed a fascinating discussion with Hulbe et al. on this topic, which is stimulating our thinking and research, and we present some of the arguments here for a wider audience. [2] In MacAyeal's model, thermal cycling in the Hudson Strait region of the Laurentide Ice Sheet caused an alternation between long-lived ice sheet growth and short-lived ice stream draw-down. The main evidence used by Hulbe et al. [2004] against MacAyeal [1993a, 1993b] is that each Heinrich event followed widespread cooling around the North Atlantic and so was not solely controlled by the "clock" of the Laurentide Ice Sheet, that thermal processes in ice streams tend to slow their motion and so would prevent the massive ice output needed for Heinrich-layer formation, and that there exists no strong evidence for the large changes in ice sheet configuration expected from the Heinrich event surges. [3] We agree that the Heinrich events seem to have consistently followed regional climatic cooling, but it is easy to have a faster process triggering a MacAyealian oscillator only when it is ready. Indeed, we have proposed more than one such mechanism for cooling triggering Heinrich events [Alley et al., 1996, 2005], and we are confident that additional triggering mechanisms are possible [e.g., Alley, 1991]. [4] Thermal controls on ice stream persistence do exist, but as shown by studies (including one involving Hulbe [Parizek et al., 2003]), subglacial water flow from beneath inland regions of ice sheets can both overcome this negative feedback and contribute to the debris entrainment necessary to explain the distal deposits of Heinrich events by allowing freezing of debris-bearing ice from the flowing water before any freezing to the bed. Indeed, the existence of high-speed ice streams and of thermal limits is central in the MacAyeal [1993a, 1993b] model. [5] Hulbe et al. [2004, paragraph [56]] also questioned the MacAyeal model because they found "no obvious evidence" for "extraordinary draw-down events" of the Laurentide Ice Sheet (LIS). Yet evidence exists in the Labrador Sector of the LIS for two sector-wide reorganizations of ice flow between the Last Glacial Maximum and final deglaciation [Dyke et al., 2002; Veillette et al., 1999]. The first reorganization indicates an eastward shift of the main center of ice outflow by up to 900 km, with an associated lowering of the ice surface over Hudson Bay of possibly hundreds of meters. Although this extraordinary event cannot be dated precisely, Dyke et al. [2002] suggested that it may have resulted from Heinrich event 1. [6] The dramatic collapse of the Larsen B ice shelf as described by Hulbe et al. [2004] and the subsequent speed-up of grounded ice behind [De Angelis and Skvarca, 2003] are of great importance in assessing stability of past and future ice sheets. However, we are cautious about applying this mechanism to the Heinrich events as done by Hulbe et al. Our first doubt is simple and indeed is noted by Hulbe et al. [2004]. The Larsen B mechanism requires cooling to grow an ice shelf, followed by warming at least in summer to fill ice-shelf surface crevasses with meltwater and may require oceanic warming to weaken the ice shelf from below [Shepherd et al., 2003]. However, all available paleoclimatic records of which we are aware show that the distinctive sedimentary signatures of the Heinrich events began during cold intervals with no premonitory warming. Unless some unobservable short warm periods triggered Heinrich events, we have difficulty reconciling the model with the data. [7] This requirement of warming to trigger the Hulbe et al. [2004] ice-shelf collapse yet lack of premonitory warming in the paleoclimatic record is probably the biggest difficulty for their model, but other considerations also serve to cast doubt. For example, if an ice shelf covered broad areas proximal to Hudson Strait prior to Heinrich events, we expect that severe reduction or even elimination of planktonic foraminifera from the surface waters would have resulted. A spike in planktonic numbers prior to Heinrich event 2 in cores from the SE Baffin Island slope, however, would suggest the opposite [Jennings et al., 1996], placing limits on the size of any such ice shelf. Indeed, the dates that have been reported on the timing of Heinrich events in the Labrador Sea and Baffin Bay are on planktonic foraminifera [Andrews et al., 1998; Hillaire-Marcel et al., 1994; Jennings et al., 1996]. These dates overlap those for the onset of Heinrich events obtained from the more distal area of the North Atlantic [e.g., Bond et al., 1992, 1999; Chapman and Shackleton, 1999], thus indicating that at least seasonally ice-free conditions existed near the margin of the ice sheet at the time of the events. [8] We further note that comparison of Greenland ice core records to marine sediment records [e.g., Bond et al., 1993] indicates that many stadials lacking Heinrich layers were as cold as stadials with Heinrich layers. This is expected from the original MacAyeal model modified to allow for triggering by marginal processes (if the bed of the Hudson Strait ice stream was still frozen, no surge could be triggered [Alley and Clark, 1999]), whereas the ice-shelf mechanism has greater difficulty providing an explanation why some very cold times grew ice shelves that collapsed to make Heinrich layers, but other very cold times followed by abrupt warmings did not produce Heinrich layers. [9] An important observation is that the part of each Heinrich layer proximal to the Laurentide Ice Sheet and Hudson Strait is dominated by sorted sediments 50–100 cm thick, indicating large volumes of meltwater and the production of turbidites [Andrews et al., 1998; Hesse and Khodabakhsh, 1998; Hesse et al., 1997, 2004; Rashid et al., 2003] despite the evidence for persistence of climatically cold conditions during Heinrich-layer deposition. In contrast, at distal sites west of Europe, iceberg-rafted debris was being supplied to deep-sea sites [Heinrich, 1988]. Thus the Heinrich layers seem to require volumes of water normally associated with jokulhlaups, and the proposed origin of the sediments [Hulbe et al., 2004, paragraph [38]] seems improbable. We have presented one model in which a cooling event could trigger a surge and an outburst flood from beneath an ice stream such as that in Hudson Strait [Alley et al., 2005], and we expect that several other models are possible. Large incised channels are suggested on seismic profiles from the Hudson Strait seafloor and adjacent shelf [Andrews and MacLean, 2003; MacLean et al., 2001] and may relate to these postulated outburst floods. [10] In addition, any mechanism invoking ice shelves faces the difficulty that they generally serve as "debris filters," holding ice near the shore but out of contact with debris sources while providing time for basal debris to melt off before icebergs are formed. Melting may be caused by heat from sub-ice-shelf water or by the "ice pump," the pressure-melting effect that removes ice from deep-draft regions and deposits it in shallower-draft regions to reduce the potential energy of the system [Lewis, 1985]; because ice shelves thin away from their grounded debris sources, ice-pump melting preferentially attacks debris-bearing ice. Ice-shelf basal melting could be avoided by a sufficiently steep basal temperature gradient or sufficiently cold ocean water; however, to the best of our knowledge, widespread grounding-line freeze-on has not been observed across the suite of modern ice shelves studied and seems unlikely in an environment warm enough to cause surface melting triggering a Larsen B-type ice-shelf disintegration. Thus, although glaciological theory does not preclude ice-shelf mechanisms for storage and release of debris, the theory does urge caution. [11] The phasing of IRD events from different ice sheets [e.g., Grousett et al., 2000] is advanced as a major reason for an ice shelf model. However, this interpretation is based on a FennoScandanavian Ice Sheet isotopic signature [Grousett et al., 2000] that Farmer et al. [2003] showed to be nonunique. In sediments south of the North Sea the IRD most likely came from the Gulf of St. Lawrence [Farmer et al., 2003]. This suggests a phasing between the Gulf of St. Lawrence and the Hudson Strait ice streams. More definitive statements require the search for more specific, ideally unique, "fingerprints" from potential IRD sources. [12] The reader should recognize that none of these objections is fatal to the Hulbe et al. [2004] model. Diffusion and bioturbation can remove sedimentary evidence of a short-lived warming, debris does exist in ice shelves, sediment cores are not available from all possible ice-shelf locations, and recognition of ice shelves from sedimentary deposits and identification of source terranes for ice-rafted debris can be equivocal. Furthermore, it is likely that ice shelves did exist at times in some places around the Laurentide Ice Sheet and contributed to its behavior, so that a complete understanding of the ice sheet and its Heinrich events will include ice shelves. Nonetheless, we believe that the weight of evidence now available argues against a warming-induced ice-shelf-disintegration model for origin of Heinrich-event layers, and we continue to favor a (modified) MacAyeal oscillator as the best model for these important and enigmatic features. We look forward to continuing discussions with Hulbe et al. on this fascinating topic. [13] We thank James Syvitski for discussions on the processes associated with the generation of turbidites; Tina Hulbe, Doug MacAyeal, George Denton, Johan Kleman, and Tom Lowell for their interesting hypothesis and generosity in discussing the key issues; and the National Science Foundation for funding our research on paleoclimatic and paleoceanographic processes.
In an extensive program of side-scan sonar and seismic reflection profiling, bottom sampling and vibracoring, we have mapped the western Gulf of Maine between Canada and Massachusetts, from the shoreline to the 100 m isobath. The purpose of the program was, in part, to locate and evaluate sand resources on the inner shelf. Surficial sand occurs on only 7% of this formerly glaciated region, and most is located seaward of southern Maine's large beaches in Wells Embayment, Saco Bay, and off Cape Small. Sand deposits occur 1) at the lowstand position of sea level, between 50 and 60 m depth, 2) on parts of the inner shelf between 50 m and the shoreface, and 3) in the shoreface. A paleodelta of the region's largest river, the Kennebec, occurs off Cape Small. Elsewhere, the lowstand deposits are thinner ( 5 m of relief on the inner shelf and contain large quantities of material. The shoreface contains the greatest concentration of sand in each of the regions. A wedge-shaped deposit of sand overlies estuarine muddy sands in each area and is inferred to have formed during a slowdown in the rate of sea-level rise between 7.5 and 9.5 ka. The volume of shoreface sand varies from less than 60 million cubic meters in Saco Bay to more than 300 million cubic meters off Cape Small, and is loosely correlated with the erosional state of adjacent beaches.
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