Patricia L. Manley

Productivity cycles of 200–300 years in the Antarctic Peninsula region: Understanding linkages among the sun, atmosphere, oceans, sea ice, and biota

Compared to the rest of the world9s oceans, high-resolution late Holocene paleoclimatic data from the Southern Ocean are still rare. We present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the Antarctic Peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr. Analyses completed include measurement of magnetic susceptibility and granulometry, bed thickness, particle size, percent organic carbon, bulk density, and microscopic evaluation of diatom and benthic foraminiferal assemblages and abundances. Downcore variability of these parameters demonstrates the significance of both short-term cycles, which recur approximately every 200 yr, and longer term events (≈2500 yr cycles) that are most likely related to global climatic fluctuations. In the upper 600 cm of the core, lower values of magnetic susceptibility (MS) are correlated with lower bulk density, the presence of thinly laminated units, specific diatom assemblages, and generally higher total organic carbon content. Below 600 cm, magnetic susceptibility is uniformly low, though variability in other parameters continues. The magnetic susceptibility signal is controlled primarily by dilution of ferromagnetic phases with biosiliceous material. This signal may be enhanced further by dissolution of magnetite in the magnetic susceptibility lows (high total organic carbon). The role of variable primary productivity and its relationship to paleoclimate is assessed through the diatom data. In particular, magnetic susceptibility lows are characterized by higher than normal abundances of Chaetoceros resting spores. Corethron criophilum and/or Rhizosolenia spp. also are found, as is a higher ratio of the most common species of Fragilariopsis versus species of Thalassiosira . These assemblages are indicative of periods of high primary productivity driven by the presence of a meltwater stabilized water column. The 200 yr cyclicity noted in other paleoclimatic records around the world suggests a global forcing mechanism, possibly solar variability. In addition to the cyclic changes in productivity, overall elevated productivity is noted below 600 cm, or prior to ca. 2500 yr B.P. This increased productivity may represent the tail end of a Holocene climatic optimum, which is widely recognized in other parts of the world, but as yet is poorly documented in Antarctica.

Marine sediment record from the East Antarctic margin reveals dynamics of ice sheet recession

The Antarctic shelf is traversed by large-scale troughs developed by glacial erosion. Swath bathymetric, lithologic, and chronologic data from jumbo piston cores from four sites along the East Antarctic margin (Iceberg Alley, the Nielsen Basin, the Svenner Channel, and the Mertz-Ninnis Trough) are used to demonstrate that these cross-shelf features controlled development of calving bay reentrants in the Antarctic ice sheet during deglaciation. At all sites except the Mertz-Ninnis Trough, the transition between the Last Glacial Maximum and the Holocene is characterized by varved couplets deposited during a short interval of extremely high primary productivity in a fjordlike setting. Nearly monospecific layers of the diatom Chaetoceros alternate with slightly more terrigenous layers containing a mixed diatom assemblage. We propose that springtime diatom blooms dominated by Chaetoceros were generated within well-stratified and restricted surface waters of calving bays that were influenced by the input of iron-rich meltwater. Intervening post-bloom summer-fall laminae were formed through the downward flux of terrigenous material sourced from melting glacial ice combined with mixed diatom assemblages. Radiocarbon-based chronologies that constrain the timing of deposition of the varved sediments within calving bay reentrants along the East Antarctic margin place deglaciation between ca. 10,500–11,500 cal yr B.P., post-dating Meltwater Pulse 1A (14,200 cal yr B.P.) and indicating that retreat of ice from the East Antarctic margin was not the major contributor to this pulse of meltwater.

Seismic Facies and Late Quaternary Growth of Amazon Submarine Fan

The Amazon Fan contains sedimentary/acoustic sequences characteristic of many large and small modern mud-rich fans. Analyses of high-resolution single-channel seismic-reflection profiles and 3.5-kHz profiles suggest that fan growth is in part related to sea-level fluctuations and in part related to events such as channel bifurcations and large debris flows that appear unrelated to sea-level position. Sinuous fan channels are perched on top of lens-shaped overbank deposits to form channel-levee systems in the upper and middle fan. Individual channel-levee systems overlap and coalesce to build levee complexes that also stack and overlap, but that are bounded by large debris-flow deposits. Because both channel-levee systems and debris flows can be active at the same time, this depositional pattern does not necessarily develop as a result of sea-level change. The sinuous fan channels appear to be nearly at grade because channel sinuosity varies downfan to keep the along-channel gradient uniformly decreasing downfan. Flat-lying, high-amplitude reflection packets that underlie a channel-levee system and extend downfan to form part of the lower fan may develop when new, oversteepened channels are created as a result of avulsion on the middle fan. This suggests that portions of the lower fan are formed concurrently with channel-levee systems. Piston cores from near the most recently active channel suggest that the locus of sedimentation shifted landward as sea level rose at the end of the last glaciation.

Cyclic Sediment Deposition Within Amazon Deep-Sea Fan

The upper and middle Amazon Fan has grown in a cyclic fashion. An individual depositional cycle consists of (1) a widespread basal, acoustically transparent seismic unit (interpreted as debris-flow deposits) that fills and levels preexisting topographic lows, and (2) a levee complex built of overlapping channel-levee systems. Two and possibly three cycles have been identified within the Amazon Fan. The levee complex beneath one debris flow originated from a different submarine canyon than did the levee complex above the debris flow, suggesting that these levee complexes formed during different sea level lowstands. Calculations based on present sediment discharge of the Amazon River suggest that an entire levee complex can form within the time span of a single glacial stag , such as the Wisconsin; however, the levee complex probably could not have formed during the relatively short time interval when sea level rose rapidly at the end of a glacial stage. The basal seismic units (debris-flow deposits) may have been deposited at any time during sea level fluctuations. Although seismic evidence suggests that this cyclic sedimentation pattern may be related to glacio-eustatic sea level variations, cyclic fan growth may be attributed to other processes as well. For example, a bottom-simulating reflector (BSR) observed within the upper fan appears to be a gas hydrate. Migration of the hydrate phase boundary during sea level fluctuations and diapiric activity may be mechanisms for initiating widespread debris flows.

Morphology of abyssal mudwaves at project MUDWAVES sites in the Argentine Basin

Abstract Extensive fields of mudwaves are present in the Argentine Basin. Mudwaves and water parameters at three sites were studied to provide detailed information on bathymetry, mudwave structure, sediment parameters and bottom water flow characteristics. Two sites, 5 and 6, are discussed in this paper. Crests of mudwaves at Site 5 on the north flank of the Zapiola Drift are oriented 45° anti-clockwise from the measured westward flow direction and migrate to the left of the flow direction. Mudwaves at Site 6 on the south flank of the Zapiola Drift are oriented 20° anti-clockwise from the inferred southeast bottom flow direction, and also migrate to the left of direction. Evidence from current meters and surface sediments suggests that the mudwaves at Site 5 are active today, while those at Site 6 have been active in the Holocene, but perhaps not within the last 100 years. Mudwave orientation anti-clockwise to mean flow, preferential sediment accumulation on the left wave flank, and apparent sediment accumulation on the wave crest agree with a model for mudwave growth presented by Blumsack and Weatherly (1989, Deep-Sea Research, 36, 1327–1339). Cross-wave asymmetry in sediment accumulation pattern is sufficient to cause a 10% change in surface sediment bulk density over the wave profile. The record of mudwave migration obtained from Sites 5 and 6 in the southern Argentine Basin and from Site 7 in the northern Argentine Basin suggests that bottom water flow has changed over a number of time scales during the last 200,000 years. Thus the analysis of abyssal bed forms can provide independent evidence about past changes in bottom flow.

High-resolution Holocene climate record from Maxwell Bay, South Shetland Islands, Antarctica

The highest resolution Holocene sediment core from the Antarctic Peninsula to date was collected during the first SHALDRIL cruise (NBP0502). Drilling yielded a 108.2-m-long core (87% recovery; site NBP0502–1B) from Maxwell Bay, South Shetland Islands. This high-resolution sediment record comes from a region that is currently experiencing dramatic climate change and associated glacial retreat. Such records can help to constrain the nature of past climate change and causal mechanisms, and to provide a context for evaluating current climate change and its impacts. The base of the drill site sampled till and/or proximal glacimarine sediments resting directly on bedrock. Glacimarine suspension deposits composed of dark greenish gray silty mud with variable diatom abundance and scattered very fine sand laminations make up the majority of the sedimentary section. Detailed sedimentological and geochemical analyses, including magnetic susceptibility, total organic carbon (TOC) content, carbon and nitrogen isotopic composition, pebble content, and biogenic silica content, allow subdivision of the glacimarine section into nine units, and seismic facies analyses resulted in the identification of six distinct seismic units. We used 29 radiocarbon ages to construct an age model and calculate sedimentation rates that vary by two orders of magnitude, from 0.7 mm/a to ?30 mm/a. Radiocarbon ages from glacimarine sediments just above the till date back to between 14.1 and 14.8 ka. Thus, ice was grounded in the fjord during the Last Glacial Maximum and eroded older sediments from the fjord. Following initial retreat of grounded ice from Maxwell Bay, the fjord was covered by a permanent floating ice canopy, probably an ice tongue. The highest sedimentation rate corresponds to an interval that contains abundant sand laminations and gravelly mud intervals and likely represents a melt-out phase or period of rapid glacial retreat from 10.1 ka to 8.2 ka. There is no evidence for an early Holocene climatic reversal, as recorded farther south at the Palmer Deep drill site. Minimum sea-ice cover and warm water conditions occurred between 8.2 and 5.9 ka. From 5.9 to 2.6 ka, there was a gradual cooling and more extensive sea-ice cover in the bay. After 2.6 ka, the climate varied slightly, causing only subtle variation in glacier grounding lines. There is no compelling evidence for a Little Ice Age readvance in Maxwell Bay. The current warming and associated glacial response in the northern Antarctic Peninsula appears to be unprecedented in its synchroneity and widespread impact.

Mudwaves on the Gardar sediment drift, NE Atlantic

Small-amplitude (<15 m) mudwaves were mapped on the Gardar Sediment Drift using the Hydrosweep DS multibeam echosounder system on the R/V Maurice Ewing. Improved data processing techniques allowed the swath mapped bathymetry to image features smaller than the nominal resolution (∼14 m) of Hydrosweep. Mudwave heights, wavelengths, and orientations were determined from the Hydrosweep data and in conjunction with 3.5-kHz profiles, these mudwave fields were analyzed with regard to present models of mudwave formation. Mudwave crest orientation was clockwise to mean bottom flow direction and migration was to the right of measured currents for two areas on the drift. This is consistent with models developed on large-amplitude mudwaves. Using mudwave orientation and migration direction in a third area, bottom current directions were inferred.

Paleoflow history determined from mudwave migration: Argentine Basin

Abstract A mudwave site in the central Argentine Basin was investigated to determine its present and past mudwave activity. Site 7, on the south flank of the Ewing Drift in the northern Argentine Basin, has mudwaves that have migrated primarily throurgh preferential deposition, resulting in a relatively complete sediment on both wave flanks. Thus a history of mudwave migration can be determined for this site. The mudwave studied shows migration from before 420 ka until 32 ka, with paleocurrent speeds near 15 cm s−1 and with minor changes in current speeds around 110–120 ka. The wave migration rate became less from 32 ka to 26 ka with paleospeeds of 9 cm s−1. Wave migration ceased about 26 ka, suggesting a cessation in flow. This study indicates that bottom water velocities at this site were high during the last glacial period. but dropped abruptly near the end of the last glacial period. However, bottom current velocities appear to have been moderately high during the previous interglacial period (∼ 120 ka), suggesting that there is no simple relationship between flow speed and paleoclimate at this site.

Andvord drift: A new type of inner shelf, glacial marine deposystem from the Antarctic Peninsula

Hemipelagic, sediment drift deposits have been discovered and mapped on the Antarctic Peninsula shelf in 300–500 m water depth. The drift located adjacent to Andvord Bay covers 44.5 km 2 and exhibits continuous and discontinuous parallel reflections that conform to peaks and valleys in the acoustic basement as observed in deep-tow boomer and sparker seismic records. This style of drift deposit is a common feature of deep oceanic sediments, but is not normally found in continental shelf environments. Measured sedimentation rates of 1–3 mm/yr on the Andvord drift indicate that the total 40 m drift thickness observed in the seismic records is probably postglacial. The drift contrasts with the basin-fill style of sedimentation that is normally associated with the Antarctic continental shelf and may play an important role in the carbon cycle. On the basis of an isopach map of drift sediments and previously published core information, the rate of carbon accumulation in the Andvord drift is estimated to be about 1.7 g/cm 2 /k.y., which is comparable to the highest rates reported for the southwestern Ross Sea.

Lakebed Pockmarks in Burlington Bay, Lake Champlain: I. Hydrodynamics and Implications of Origin

Using side-scan sonar and sub-bottom profiles, a 40 m diameter, ~4 m deep lakebed pockmark was investigated in Burlington Bay, Lake Champlain, Vermont. Five piston cores (2 inside, 1 on the rim and 2 outside of the pockmark) were collected and analyzed for magnetic susceptibility, physical properties and 210Pb and 132Cs concentrations. A yearlong subsurface mooring, equipped with an Acoustic Doppler Current Profiler (ADCP) and vertical temperature chain, was placed in the deepest section of the pockmark (27.7 m). A second temperature chain was placed along the pockmark floor and over its rim. At the base of the mooring, 35 mm stereo cameras and a Savonius rotor current meter ~ 0.3 m above the lake floor) were installed. Bottom photographs show episodic events of suspended sediment. Temperature data show the presence of anomalously cold temperature water near the rim of the pockmark during stratified periods and anomalously warmer water temperatures during isothermal winter type conditions. Coarser grain size and bulk density occur inside the pockmark with an associated decrease in water content in comparison to sediment outside the pockmark. Radionuclide results show no detectable net accumulation of sediment within the rim of the pockmark, slight accumulation in the deepest part of the pockmark, and only a few tenths of a mm/yr accumulation outside. This pockmark was most likely formed by groundwater seepage and is subsequently modified by continuing groundwater seepage and bottom-flowing currents. Taylor column and frictional boundary layer dynamics are believed to play a role in the circulation within this feature.