Present-day Circulation Pattern Research Articles

Late Holocene current patterns in the northern Patagonian fjords recorded by sediment drifts in Aysén Fjord

Present-day circulation patterns in the southeastern Pacific Ocean are driven by the Antarctic Circumpolar Current, directing subantarctic surface water into the Patagonian fjords since at least the early Holocene. In this way, bottom current patterns in the area are regulated by the regional climate, although the complex bathymetry of the fjords has a significant impact as well. To understand the potential interplay of climate, seafloor topography and circulation patterns, we study the sedimentary infill of Aysén Fjord (~45°S) and reveal the first active sediment drifts in the region. These allow constraining the present-day circulation patterns in northern Patagonia and show an incoming (southward) as well as returning (northward) flow direction. While the general sedimentary evolution of the fjord (and thus also the sediment drifts) is climate-driven (i.e., it reflects variability in southern westerly wind strength), the onset of drift formation at ~3.7 ka does not seem to have originated from an abrupt change in regional climate. Instead, we propose that a megathrust earthquake described in paleoseismic records in the area could have resulted in subsidence of one (or more) of the many bathymetric highs in the Patagonian fjords, thus contributing to enhanced spilling of subantarctic water into the fjord. This study underscores the importance of multidisciplinary research to understand past and present bottom current circulation patterns and disentangle different possible feedback mechanisms.

Santos Drift System: stratigraphic organization and implications for late Cenozoic palaeocirculation in the Santos Basin, SW Atlantic Ocean

Abstract High-quality 2D and 3D seismic data were analysed to investigate the stratigraphic organization of the sedimentary deposits and the impact of the palaeocirculation of the SW Atlantic Ocean in the construction of the Santos Basin slope from late Palaeogene to Recent time. Seven seismic sequences were identified based on their external geometry, internal seismic pattern and seismic boundaries. A correlation between these sequences, the glacio-eustatic curves and the major climatic-palaeoceanographic events was attempted. The base of the studied sequences is the c. 28 Ma intra-Oligocene Rupelian-Chatian unconformity. Six other seismic horizons corresponding to the sequence boundaries were mapped ranging from the late Oligocene to the Pliocene. Variations in the bottom-current intensity were characterized by the seismic pattern of each sequence and by the evolution of margin physiography. A 100 km long channel-like gutter, the Santos Channel, was excavated at the foot of an intra-slope escarpment. It was the locus of major flow circulation until the middle Miocene, when the margin physiography reorganization transferred the main axis of deep current action downslope and excavated the São Paulo Channel at the foot of the present continental slope. Two major contourite drifts were accumulated in response to the different combinations of bottom-current axis position and slope physiography. Those drifts constitute the Santos Drift System. Conversely to the present-day circulation pattern, with southward flow above the slope dominated by the western boundary Brazil Current, both at surface and deep waters, the geological record indicates that the palaeocirculation in the Santos Basin was marked by the opposite sense of circulation of surface waters (southward palaeo Brazil Current) and of the intermediate to deep waters (northward Southern Ocean Current). The study indicates that periods of relative sea-level rise to highstands correspond to increase in drift accumulation whereas during lowstands slope drift sedimentation is reduced.

Amphi-Atlantic phylogeography of direct-developing lineages of Lasaea , a genus of brooding bivalves

Direct-developing intertidal Lasaea spp. occur in the North Atlantic as both continental margin and oceanic island populations. We conducted a molecular phylogenetic analysis of representative populations in order to test colonization hypotheses for North Atlantic oceanic islands. Thirty individuals each were collected in 1995 and 1996 from two continental putative source populations (Florida, Iberia) and two oceanic island populations (Bermuda, Azores). They were sequenced for a 462 nucleotide portion of the mitochondrial large ribosomal subunit (16S) gene. No amphi-Atlantic genotypes were detected: Bermudan lineages co-clustered exclusively with Floridian congeners, and Azorean samples formed an exclusive clade with Iberian haplotypes. Our data indicate that geographical proximity to continental source populations is a better predictor of phylogenetic relationships in North Atlantic Lasaea spp. than present-day oceanic surface circulation patterns. The phylogenetic trees generated are not consistent with colonization of oceanic islands by indirect-developing ancestral lineages or by truly trans-oceanic rafting events. However, they are consistent with predicted topologies resulting from limited (≤ 2000 km), long-distance colonization by rafting (against present-day circulation patterns in the case of the Azores) and from anthropogenic introductions.

Eolian, fluvial and pan (playa) facies of the Tertiary Tsondab Sandstone Formation in the central Namib Desert, Namibia

The Pliocene to Recent sand sea in the central part of the 2000 km long Namib Desert is underlain widely by sediments comprising the Tsondab Sandstone Formation (TSF) of Early to Middle Tertiary age. This formation is the earliest record of terrestrial sedimentation in the central Namib Desert and formerly was thought to consist of only eolian dune and sand sheet deposits of a desert origin. However, recent field investigations indicate that the TSF also includes deposits of ephemeral watercourses and pans (playas). These fluvial and playa sediments are associated intimately with the eolian dune and sand sheet deposits, as is likewise the case in the present, extreme-arid Namib Desert. Sedimentary structures and trace fossils in the TSF eolianites resemble those found in the modern sand sea. Furthermore, these eolianites were deposited under a dominant southerly and a subordinate easterly quadrant wind regime which is similar to the present-day circulation pattern in the Namib. Since the cold Benguela Current was established only in the Late Tertiary, the TSF represents a major, ancestral arid phase in the Namib tract that was controlled by an early anticyclonic wind regime and a geographic location on the rainshadow side of southern Africa.

Low-salinity surface water incursions across the Strait of Sicily during late Quaternary sapropel intervals

Sapropel layers (black, organic-rich sediments) in the deep Eastern Mediterranean Basin are indicators of anoxic conditions that occurred intermittently during the Pleistocene. During these times of deep-water stagnation, the surface waters were marked by changes in planktonic foraminiferal assemblages indicative of reduced salinity. Previous work has established a strong link between low-salinity surface waters and glacial meltwater inflow into the Eastern Mediterranean via the Black Sea. Discriminant analysis was applied to foraminiferal abundance data from the sapropelcontaining core, Lynch II-3, collected in 2400 m water depth approximately 80 km east of the Strait of Sicily. The function produced is 98% effective at distinguishing the faunal assemblages in non-sapropel sediments from those associated with four sapropel layers in this core. The most distinctive characteristic of the sapropel faunas is a high percentage of Neogloboquadrina dutertrei. The discriminant function has been applied to a suite of cores from the Strait of Sicily that lack sapropels because the strait (maximum depth of 1700 m) has remained continuously aerated. This analysis has distinguished planktonic foraminiferal assemblages and associated sediments coeval with sapropel formation in the deeper eastern basin. These correlations are supported by planktonic foraminiferal biostratigraphy and a distinctive ash layer in core Lynch II-3 and two cores from the Strait. Existence of the sapropel assemblage in core TR171-11 from the western entrance of the strait indicates that low surface salinities extended through the strait, a condition requiring a surface water flow from east to west, a reversal of the present-day circulation pattern.

Correlation of Monterey Shale to Paleo-Oceanographic and Paleoclimatic Events: ABSTRACT

Three distinct oxygen isotope events, which appear to record successive stages in the growth of the east Antarctic ice sheet, have been identified and dated paleomagnetically in middle and upper Miocene sediments from the Pacific Ocean. These stages are as follows: (1) late Magnetic Epoch 15 to early Magnetic Epoch 12 (c. 15 to 12 m.y.B.P.); (2) early Magnetic Epoch 11 (c. 11 m.y.B.P.); and (3) early Magnetic Epoch 6 (c. 6.7 m.y.B.P.). An additional slight cooling is recorded in Magnetic Epoch 10 at c. 10 m.y.B.P. These events can be tied to changes in paleo-oceanography, paleobiogeography, and opal accumulation in the Pacific Ocean, and, in turn, can be identified in the Monterey. The initiation of the east Antarctic ice sheet at c. 14 to 12 m.y.B.P., and its attendant c anges in surface circulation and paleoproductivity, is correlated to the beginning of the Monterey Shale. In addition to the beginning of massive opal accumulation along the California coast, increased silica accumulation in the equatorial Pacific is noted. This is accompanied by changes in diatom communities marking the beginning of present-day circulation patterns. The role of the Southern Ocean as an arbiter in the Miocene and Pliocene silica budget is noted and discussed. End_of_Article - Last_Page 908------------