Philip J. Bart

Timing and progression of climatic, tectonic and glacioeustatic influences on the Messinian Salinity Crisis

Abstract New palynological analysis of the continuous Upper Neogene sequence from the Rifian Corridor at Sale (Morocco) permits the deconvolution of climatic, tectonic and eustatic control on the Messinian Salinity Crisis via two indices: (1) a ‘distance-from-shore’ index based on dinoflagellate cyst versus pollen distribution, and (2) a land-climate index based on detailed pollen analysis. This new pollen analysis indicates that the Messinian Salinity Crisis (∼6.8 Ma to ∼5.3 Ma) was not associated with major climate change. Detailed analyses of dinoflagellate cyst ecology at Sale correlated to the Sale δ18O record show that initial shoaling of the Rifian Corridor after ∼7.3 Ma primarily resulted from tectonic uplift. However, from ∼6.5 Ma to ∼5.4 Ma, the Rifian Corridor was sufficiently shallow to have experienced significant restriction of Atlantic inflow during at least four small-amplitude glacioeustatic lowstands. At ∼5.4 Ma, abrupt increase in the ratio of continental- versus marine-derived palynomorphs indicates that restriction intensified as tectonic uplift significantly outpaced gradual glacioeustatic rise. Despite eustatic rise at ∼5.32 Ma, the Sale palynological data show that the Rifian Corridor experienced a second major drop in relative sea level (tectonic uplift or increased sedimentation). Therefore, termination of the Messinian Salinity Crisis was probably associated with a breach elsewhere along the Gibraltar Arc.

Relative temporal stability of the Antarctic ice sheets during the late Neogene based on the minimum frequency of outer shelf grounding events

Abstract The stability of the Antarctic cryosphere involves two fundamental aspects: the magnitude of ice volume changes, and the frequency of ice volume changes. In this article, we synthesize results from three regional seismic stratigraphic studies of the continental shelf to evaluate the relative frequency of extreme advances of the ice sheets during the late Neogene for different sectors of the Antarctic ice sheet. Detailed analyses of glacial unconformities on the eastern and western Ross Sea outer continental shelf indicate that there were at least eight episodes during the late Neogene when the East Antarctic ice sheet (EAIS) and the West Antarctic ice sheet (WAIS) were significantly larger. The seismic results from the shelf do not support the conventional view that the land-based EAIS was relatively stable and that the marine-based WAIS was relatively dynamic. Glacial unconformities on the Pacific continental shelf of the Antarctic Peninsula indicate that there were at least 30 outer shelf ice sheet expansions during the late Neogene. This suggests that the small land-based Antarctic Peninsula Ice Cap (APIC) may have been the most dynamic component of the Antarctic cryosphere. Conversely, the greater number of grounding events on the Antarctic Peninsula shelf may be a result of the margin’s younger thermal age and hence greater potential to preserve units and unconformities during the late Neogene. We acknowledge that the available chronostratigraphic control from the Antarctic margin is poor, and that more chronological data are needed to confirm our late Neogene age assignments.

Were West Antarctic Ice Sheet grounding events in the Ross Sea a consequence of East Antarctic Ice Sheet expansion during the middle Miocene

Abstract Seismic correlation of glacial unconformities from the Ross Sea outer continental shelf to chronostratigraphic control at DSDP sites 272 and 273 indicates that at least two West Antarctic Ice Sheet (WAIS) expansions occurred during the early part of the middle Miocene (i.e. well before completion of continental-scale expansion of the East Antarctic Ice Sheet (EAIS) inferred from δ18O and eustatic shifts). Therefore, if the volume of the EAIS was indeed relatively low, and if the Ross Sea age model is valid, then these WAIS expansions/contractions were not a direct consequence of EAIS expansion over the Transantarctic Mountains onto West Antarctica. An in-situ development of the WAIS during the middle Miocene suggests that either West Antarctic land elevations were above sea level and/or that air and water temperatures were sufficiently cold to support a marine-based ice sheet. Additional chronostratigraphic and lithologic data are needed from Antarctic margins to test these speculations.

Did the Antarctic ice sheets expand during the early Pliocene

Seismic data show that glacial unconformities are located within lower Pliocene strata on the Antarctic continental shelves. The glacial unconformities are significant because they provide direct evidence that the Antarctic ice sheets advanced despite the generally warmer climates and elevated sea levels that characterized most of the early Pliocene. The magnitudes of peak eustatic lowstands and 18 O enrichments indicate that the ice volume on Antarctica may have exceeded today9s ice volume by approximately 18%, which suggests that the ice-sheet grounding events on the shelves probably were associated with larger than present ice volumes on two to three occasions during the early Pliocene.

West Antarctic Ice Sheet grounding events on the Ross Sea outer continental shelf during the middle Miocene

Abstract New seismic–stratigraphic analysis of the Ross Sea continental shelf suggests that there were a at least five shelf-wide grounding events of the West Antarctic Ice Sheet (WAIS) during the middle Miocene. Although the number of WAIS grounding events generally matches the number of extreme δ 18 O enrichments and eustatic lowstands, these results do not support the long-standing assumption that West Antarctica was substantially ice-free. Instead, seismic–stratigraphic evidence from the Ross Sea shelf documents waxing and waning of a well-developed WAIS in the marine environment at least on the Pacific sector of the West Antarctic continental shelf.

PALYNOLOGY OF THE NBP03-01A TRANSECT IN THE NORTHERN BASIN, WESTERN ROSS SEA, ANTARCTICA: A LATE PLIOCENE RECORD

Abstract Fifty-seven samples taken from ten piston cores collected along a transect off the continental margin of the Northern Basin, Ross Sea, Antarctica were analyzed for palynomorphs. Moderately diverse assemblages of marine microplankton and terrestrial palynomorphs were recovered. The palynomorph assemblages have been subdivided into two main groups: the in-situ flora (including acritarchs, dinoflagellate cysts, leiospheres and prasinophyte algae mainly composed of cymatiosphaerids), and the reworked flora (including dinoflagellate cysts, pollen and spores). The leiospheres are the most abundant palynomorphs. This prominence in the relative abundance of leiospheres has been reported as typical of assemblages found today at the limit between seasonal and pack ice in the Arctic. In-situ dinoflagellate cysts are sparse. They are mainly represented by Lejeunecysta, which, based on species similarities to those from Cape Roberts, are believed to be of Oligocene to Pliocene age. All other dinoflagellate cysts recovered are the result of reworking from Eocene to Oligocene sediments. Reworked spores and pollen comprise the second most abundant group. They are of moderate diversity and include an Eocene or older assemblage of Nothofagidites, Podocarpaceae and Proteaceae. Other taxa are representative of warmer rainforest vegetation, with Oligocene and Neogene taxa that include representatives of woodland to herbaceous/low shrubby tundra vegetation growing in colder subpolar climates. These assemblages indicate either different periods of deposition or reworking from diverse sources. Through seismic correlation and diatom analysis, the sediments are believed to be Late Pliocene in age. On this basis, it is postulated that the major glacial advance, RSU 2 of Brancolini et al. (1995) or Unconformity 10 (U10) of Bart et al. (2000), occurred before 2.3 Ma, which is the oldest age of in-situ species recovered in units above U10. As both terrestrial and marine reworked taxa include assemblages of Eocene to Oligocene age, it is assumed that those reworked components were yielded from a single source; most probably Eocene to Oligocene shallow marine strata eroded and transported from the area of Ross Island to the shelf margin through ice streams located in the Drygalsky and Joides basins.

The paradox of a long grounding during West Antarctic Ice Sheet retreat in Ross Sea

Marine geological data show that the West Antarctic Ice Sheet (WAIS) advanced to the eastern Ross Sea shelf edge during the Last Glacial Maximum (LGM) and eventually retreated ~1000 km to the current grounding-line position on the inner shelf. During the early deglacial, the WAIS deposited a voluminous stack of overlapping grounding zone wedges (GZWs) on the outer shelf of the Whales Deep Basin. The large sediment volume of the GZW cluster suggests that the grounding-line position of the paleo-Bindschadler Ice Stream was relatively stationary for a significant time interval. We used an upper bound estimate of paleo-sediment flux to investigate the lower bound duration over which the ice stream would have deposited sediment to account for the GZW volume. Our calculations show that the cluster represents more than three millennia of ice-stream sedimentation. This long duration grounding was probably facilitated by rapid GZW growth. The subsequent punctuated large-distance (~200 km) grounding-line retreat may have been a highly non-linear ice sheet response to relatively continuous external forcing such as gradual climate warming or sea-level rise. These findings indicate that reliable predictions of future WAIS retreat may require incorporation of realistic calculations of sediment erosion, transport and deposition.

Grounding Zone Wedges on the Antarctic Continental Shelf, Antarctic Peninsula

The Antarctic Peninsula accommodates a small volume of ice but ice-volume fluctuations were sufficiently large to repeatedly advance the grounding line and associated grounding zone wedges across the shelf. Grounding zone wedge stratigraphy records the history of glaciation on the shelf. The continental shelf is approximately 150 km wide and the shelf break averages 400 m. On the shelf, a series of glacially scoured troughs dominate the bathymetry.

A palynological and sequence-stratigraphic study of Santonian–Maastrichtian strata from the Upper Magdalena Valley basin in central Colombia

This paper presents a sequence-stratigraphic interpretation from the palynological analysis and lithologic data of two outcrop sections on the NE flank of the Upper Magdalena Valley (UMV) basin primarily comprising the Santonian—Lower Maastrichtian interval. Important stratal horizons are identified in the northeastern part of the UMV basin and ages assigned to them. A cyclic pattern of palynomorph distribution was recognized in both sections and tied to the different stages of the stratigraphic chart. Spikes in abundance of spores accompanied by pollen characterize the lowstand systems tracts and are replaced by the occurrence of euryhaline dinoflagellate cysts (ceratioids and/or gymnodinioids) during the subsequent transgressive phase. Maximum flooding surfaces (MFS) are recognized by a sudden increase in open marine palynomorphs (peridinioids and/or gonyaulacoids) and the scarcity of terrestrial representatives. As sea level starts to fall, the gradual decrease in open marine dinoflagellates along with the occurrence of euryhaline dinoflagellate cysts and terrestrial representatives corresponds to highstand systems tracts. The sequence-stratigraphic interpretation from palynological analysis was correlated to the global sea-level curve allowing the identification of the Santonian—Campanian and Campanian—Maastrichtian boundaries. System tracts from supercycles ZC-3, ZC-4 and TA-1 were recognized from the palynological data.

Trough-mouth fans: Crary Fan, Eastern Weddell Sea, Antarctica

The “trough-mouth fan (TMF)” concept was introduced by Vorren et al. [1989] when describing the large cone-shaped glaciogenic depocentre on the Barents Sea continental margin, located near Bear Island at the mouth of a transverse shelf trough of glacial origin. Such TMF’s are composed of glacial debris deposited on the outer shelf and upper slope at the grounding line of ice streams when these extend out to the shelf edge. This glacial debris forms prograding-slope strata that pinch out in basinward direction. A comparative MCS study has allowed Vanneste [1995] to identify three basic types of TMF’s: 1. mostly stable TMF’s characterised by absence of large-scale mass-wasting deposits (e.g. Scoresby Sund TMF off East Greenland); 2. unstable TMF’s characterised by the presence of large-scale mass-wasting deposits (e.g. Bear Island TMF in Barents Sea); and 3. TMF’s associated with deep-sea fan systems in their distal parts (e.g. Crary Fan in Weddell Sea).