Grain-size distribution and significance of clay and clay-sized minerals in Eocene to Holocene sediments from Sites 918 and 919 in the Irminger Basin

Abstract

Lower Eocene to Holocene sediments recovered from Ocean Drilling Program Sites 918 and 919 were studied to determine the grain-size distribution (sand to clay sizes) and mineralogy of the <2 µm size fraction. The minerals are believed to be of detrital origin. The clay minerals consist of chlorite, smectite, illite, kaolinite, and a mixed-layer illite/smectite. Several nonclay minerals were identified as well, including quartz, plagioclase, alkali-feldspar, amphibole, pyroxene, zeolite, and calcite. Relative abundances of the clay minerals were semiquantified using an oriented internal standard. Smectite abundances were found to increase with depth, while illite and chlorite abundances decrease with depth. The Eocene sediments of Site 918 are characterized by a predominance of smectite with some kaolinite and very small amounts of chlorite and illite. This mineral assemblage is indicative of warm climatic conditions at the time of deposition. Oligocene sediments show an increase in chlorite and illite, suggesting that a sediment dam may have existed on the continental shelf, trapping these sediments and preventing their transport into the Irminger Basin, prior to this time. A warming trend in the early to middle Miocene is indicated by increased amounts of kaolinite. Variations in the relative amounts of chlorite and illite at this time may be the result of short-term eustatic sea level changes. Near the end of the Miocene, a cooling trend is indicated by increasing amounts of chlorite and illite relative to smectite abundance. This follows deposition of glauconitic hardgrounds at the top of a chalk unit with coarse sediment deposited above them, often with erosion of the underlying hardground. This interval is believed to coincide with the spillover of Arctic seawater into the Irminger Basin. The hardgrounds indicate hiatuses of short duration (thousands to tens of thousands of years) followed by high density turbidity currents or renewed chalk deposition. Non-clay minerals in the <2 µm size fraction become common in upper Miocene to Holocene sediments of Sites 918 and 919, as glacial rock-flour that signals the onset of Greenland glaciation. Using R-mode factor analysis, the clay assemblages of Pliocene-Pleistocene age sediments were found to be significantly different from older sediments, with a dramatic transition at 457 mbsf, recording the beginning of significant Greenland glaciation.

Late Pleistocene Emergence of Crystalline Canadian Shield Sources in Sediments of the Northern Gulf of Mexico

“…The earliest documented meltwater event corresponds with identified tills deposited ∼2.4 Ma just north of the present Missouri-Mississippi confluence (Balco & Rovey, 2010). In addition, the recurring presence of clay-sized plagioclase feldspars observed in this study from the MP onwards (Figure 2c) indicates the input of glacially abraded rock-flour (Heiden & Holmes, 1998;Menking, 1997) derived from fresh source rocks.…”

The late Miocene to Pleistocene ice‐rafting history of southeast Greenland

“…SE Greenland presently has very mountainous topography and receives more precipitation than any other region in Greenland (Reeh 1989), conditions that favor the development and maintenance of glaciers. It is reasonable to assume that similar conditions also prevailed in the late Miocene because: (1) the regional uplifted topography has been present since at least the early Miocene (Larsen 1990;Christiansen et al 1992; Shipboard Scienti c Party 1994b), (2) the Gulf Stream, which provides moisture to SE Greenland, began to strengthen in the late Miocene as the Isthmus of Panama began to close (Kaneps 1979;Duque-Caro 1990;Haug & Tiedemann 1998;Reynolds et al 1999), and (3) SE Greenland experienced climatic cooling in the late Miocene (Heiden & Holmes 1998;Spezzaferri 1998). Thus our data support Larsen et al's (1994) proposal that SE Greenland may have been a key nucleation area for the Greenland Ice Sheet.…”