ABSTRACT: The mineralogy of Baffin Bay seafloor surface sediment: insights into Holocene/Late Glacial changes in sediment provenance.

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Abstract

Sediments are shed on to the floor of Baffin Bay from the surrounding land masses (Fig. 1) through iceberg rafting, meltwater plumes, and the remobilization of sediments. Changes in sediment source(s) during the Holocene and latest Pleistocene were tracked by analysis of mineral composition. The bedrock geology of the sediment source areas consists mainly of a variety of igneous and metamorphic rocks of Archean to Paleoproterzoic in age, but with critical outcrops of carbonates (mainly dolomite), and a limited outcrop area of other sedimentary rocksand of early Tertiary basalts. The surface mineralogy of Baffin Bay was characterized by quantitative X-ray diffraction (qXRD) analysis of the < 2 mm sediment fraction; some 100 samples have been processed using ZnO as a calibration spike and the program “RockJock v6” (Eberl, 2003, 2004). Some 22 non-clay minerals and 10 clay mineral species were identified on the basis of their XRD patterns in the range between 5° and 65° 2-theta using Cu K-alpha radiation. Weight % estimates sum to ~100%. The bulk of the surface sediments were obtained from grab samples archived at the Geological Survey of Canada’s Core Depository, Dartmouth, NS, mainly from cruises of CSS Hudson in 1964, 1977, and 1980. Median water depth of the samples is 590 m, with a range between 350 and 2365 m. Additional parameters pertaining to mineralogy and sediment history were obtained from grain-size measurements (Malvern laser-sizer), SIRM, BIRM and magnetic susceptibility, and color derived from a spectrophotometer on dried sediment samples. Analysis of the results using a variety of multi-variate statistical methods [e.g. Principal Component Analysis PCA, Discriminat Function Analysis (DFA), trend surface, and cluster analysis] indicated that distinct spatial clusters can be defined. These insights are applied to the downcore variations in mineralogy of 9 box cores and 3 piston cores from the region. The box cores, recovered between 20-50 cm, were obtained on cruises HU2008-029 and JCR175 (2009)---they were sampled at 1-cm intervals and the piston cores at 10-cm intervals. At the moment, chronological control is limited and will be, substantially improved. Source areas were determined from application either of DFA or of the program MinUnMix (Eberl, 2004), which “unmixes” a mineralogical mixture using up to 6 end members (Fig. 2). The sediments are dominated by non-clay minerals,which average 70 ± 7 wt%. Ten mineral species constitute, on average, more than 85% of the compositions; in rank order these are: orthoclase, quartz, microcline, chert, illite. iron smectite, biotite, chlorite, and dolomite. Smaller but regionally important species include amphibole and pyroxene. An important marker in several of the box and piston cores is the disappearance of calcite (Fig. 3)---this is attributed to the onset of corrosive conditions, which, in some cores, appears to date from between 5000-6000 cal yr BP.

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