Last Glacial and Holocene Continental Slope Sedimentation on the Mozambique Margin, NE off Zambezi Delta: Preliminary Results

Author(s):  
I.H. Guambe ◽  
G. Jouet ◽  
B. Dennielou ◽  
J. Mugabe ◽  
M. Achimo ◽  
...  
2020 ◽  
Author(s):  
Carmen Argenio ◽  
Pierluigi Palladino ◽  
José Abel Flores Villarejo ◽  
Filomena Ornella Amore

<p>During the past 25 ky, the Earth system underwent a series of dramatic climate transitions until the most recent glacial period. It peaked about 21 ky ago during the time interval known as “Last Glacial Maximum” (LGM). This study focuses on the reconstruction of global changes occurred from the LGM to the Holocene.</p><p>For this aim coccolithophore assemblages have been studied at Integrated Ocean Drilling Program (IODP) Site U1385 (37°34.285’N, 10°7.562’W, 2578 m below sea level) located on the continental slope of the southwestern Iberian Margin in a timeframe between 25 and 0 ky. Moreover, an integration with isotopic and biogeochemical data and a comparison with other proxies were carried out.</p><p>This IODP Site nowadays is influenced by the Portugal Current system (Pérez et al., 2001; Relvas et al., 2007), whose seasonality is driven by migrations of the semi-permanent subtropical Azores High pressure system (Coelho et al., 2002). The study area also undergoes intra-seasonal oscillations mainly related to changes, during winter, of westerly wind prevalence, induced by the North Atlantic Oscillation (Trigo et al., 2004).</p><p>Coccolithophore data were carried out by sediments from the first four sections of the core A of the IODP Site U1385. Coccolithophores, haptophyte algae living in the photic zone, are sensitive to some environmental parameters as temperature, salinity, availability of nutrients and sunlight. Thanks to their ecological sensitivity, coccolithophores are able to record paleoceanographic changes and for this reason are considered to be an important proxy to study the climate variability.</p><p>The age model was calculated using linear interpolation between 64 tie points based on log (Ca/Ti) records of Site U1385 and MD01-2444 (Hodell et al., 2015; Datema et al., 2019) and on δ<sup>18</sup>O records of Site MD01-2444 and Greenland (Hodell et al., 2013). About 500 samples were sampled and preliminary results are based on the analysis of samples with a time-resolution of about 0,3 ky.</p><p>The preservation of the assemblages is from good to moderate (Flores et al., 2003). For quantitative analyses, a minimum of 300 coccoliths was counted per slide in a varying number of visual fields using a light microscope at 1000x magnification. This allows a 95% level of confidence to be reached for all species present in at least 1% abundance (Patterson and Fishbein, 1989). Absolute abundance (coccoliths per gram of sediment) and nannofossil accumulation rate (NAR; coccoliths cm<sup>-2</sup> ka<sup>-1</sup>) were estimated following Flores and Sierro (1997).</p><p>The preliminary results highlight a progressive increase of small Gephyrocapsa and a decrease of Emiliania huxleyi, between 4,26 ky and 0,91 ky. Moreover, most abundant species, in this interval, are Gephyrocapsa oceanica, Umbilicosphaera sibogae and Calcidiscus leptoporus. Furthermore, between 18,40 ky and 14,72 ky a significant increase of E. huxleyi > 4 µm and G. mullerae occurs associated with a decrease of small Gephyrocapsa and E. huxleyi.</p>


Author(s):  
J. Meincke

SynopsisPreliminary results from a winter and a summer cruise in 1984 to the area west of the U.K. continental slope are presented to discuss the structure and the spatial scales of convection. Winter convection events were found to reach a depth of 630 m with horizontal scales of the order of 50 km. The number of areas with actual convection to maximum depth was small at the particular time of the cruise, but the vertical structure in the investigated area indicated numerous convection events over a longer period. The principal vertical structure of the upper 600 m in winter was preserved until the following summer, which agreed with the age of the summer upper layer water estimated from the tritium/helium ratio. This characterises the area to be one of low advection, which can also be indirectly concluded from the fact that the northward flow of 4 Sverdrup of Atlantic Water through the Faeroe-Shetland Channel is supplied by 2 Sverdrup from the current over the continental slope west of U.K. and 2 Sverdrup of flow along the Arctic Front between Iceland and the Faeroes.


2019 ◽  
Vol 47 (3) ◽  
pp. 20-38
Author(s):  
A. G. Zatsepin ◽  
V. V. Kremenetsky ◽  
O. I. Podymov ◽  
A. G. Ostrovsky

The preliminary results of an experimental study of a mechanism of the Black Sea anoxic layer ventilation related with the descent of oxygen-containing water down the bottom slope in Ekman boundary layer are presented. To study this mechanism, several automatic measuring stations were installed at the bottom of the shelf-continental slope zone in the depth range from 80 to 243 m, on the cross-section abeam of the Gelendzhik Bay. The observations that lasted for 1.5–2 months were fulfilled during two periods in the beginning and the end of 2018. The stations were registering hydrophysical (temperature, salinity, pressure and current velocity) and hydrochemical (dissolved oxygen concentration) parameters at a 0.5–2.5 m distance from the bottom. The acquired data are suitable for estimation of spatio-temporal parameters of water transfer in the bottom layer up and down the slope, depending on direction and intensity of alongshore current. Preliminary analysis of the first installation data confirmed the presence of bottom water transfer along the slope perpendicular to the shore. Also, in case of intense alongshore north-western current, a descent of the bottom layer water was observed. Such manner of water transfer conforms to both geostrophic adjustment and dynamics of the bottom Ekman boundary layer. However, variability of water density in the bottom layer, caused by vertical water transport, had about the same range as density fluctuations in the water column on the same depths. This fact disputes the effectiveness of Ekman transfer in the bottom layer as a water ventilation mechanism for the upper part of continental slope of the Black Sea (Zatsepin et al., 2007; Elkin, Zatsepin, 2017).


2004 ◽  
Vol 55 (2) ◽  
pp. 131-140 ◽  
Author(s):  
David J. W. Piper ◽  
Adam Macdonald

Abstract At the last glacial maximum, the major ice outlet through Laurentian Channel terminated on the upper continental slope. A 10 km square area of the upper slope has been investigated in detail, using airgun and boomer seismic reflection profiles and piston cores. Sediment failure during the 1929 Grand Banks earthquake resulted in exposure at the seabed of Last Glacial Maximum sediments that are normally buried beneath tens of metres of younger strata. Ice-margin acoustic and lithofacies are interpreted using criteria developed on the continental shelf and chronology is provided by AMS radiocarbon dates on in situ mollusc shells. Seismic data show a morainal ridge at 500 mbsl (mbsl = metres below (present) sea level) corresponding to the Last Glacial Maximum ice grounding line. A change in thermal regime of the ice or a subglacial meltwater outburst, at 16.5 ± 0.15 ka (radiocarbon years, -0.4 ka marine reservoir correction applied), resulted in release of sediment-laden meltwater that eroded gullies on the continental slope. This erosion surface is immediately overlain by a prominent stony diamict that extends to about 700 mbsl and may represent till deposition from a glacial surge. The ice margin then retreated upslope by 16.3 ka, probably to the prominent moraine at 380 mbsl at the lip of the Laurentian Channel. Evidence from mud turbidites on Laurentian Fan suggests that this ice marginal position may have persisted until about 14.2 ka. Ice then retreated rapidly northwards up Laurentian Channel, synchronous with Heinrich Event 1 at about 14 ka. Younger proglacial sediment on the upper continental slope slumped at about 12 ka, probably as a result of loading by a late-ice advance across St. Pierre Bank.


1999 ◽  
Vol 173 ◽  
pp. 185-188
Author(s):  
Gy. Szabó ◽  
K. Sárneczky ◽  
L.L. Kiss

AbstractA widely used tool in studying quasi-monoperiodic processes is the O–C diagram. This paper deals with the application of this diagram in minor planet studies. The main difference between our approach and the classical O–C diagram is that we transform the epoch (=time) dependence into the geocentric longitude domain. We outline a rotation modelling using this modified O–C and illustrate the abilities with detailed error analysis. The primary assumption, that the monotonity and the shape of this diagram is (almost) independent of the geometry of the asteroids is discussed and tested. The monotonity enables an unambiguous distinction between the prograde and retrograde rotation, thus the four-fold (or in some cases the two-fold) ambiguities can be avoided. This turned out to be the main advantage of the O–C examination. As an extension to the theoretical work, we present some preliminary results on 1727 Mette based on new CCD observations.


2000 ◽  
Vol 179 ◽  
pp. 163-165
Author(s):  
S. K. Solanki ◽  
M. Fligge ◽  
P. Pulkkinen ◽  
P. Hoyng

AbstractThe records of sunspot number, sunspot areas and sunspot locations gathered over the centuries by various observatories are reanalysed with the aim of finding as yet undiscovered connections between the different parameters of the sunspot cycle and the butterfly diagram. Preliminary results of such interrelationships are presented.


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