“Geochemical Expression of Early Diagnesis in Middle Eocene-Lower Oligocene Pelagic Sediments In the Southern Labrador Sea, Site 647,
ODP LEGASTM D 513-88. 105.” 111-135
Arthur, M. A., W.E. Dean, J.C. Zachos, M. Kaminski, S. Hagerty Rieg, and K. Elstrom
Geochemical analyses of the middle Eocene through lower Oligocene lithologic Unit IIIC (260-518 meters below seafloor [mbsf]) indicate a relatively constant geochemical composition of the detrital fraction throughout this depositional interval at Ocean Drilling Program (ODP) Site 647 in the southern Labrador Sea. The main cariability occurs in redox-sensitive elements (e.g. iron, manganese, and phosphorus), which may be related to early diagenetic mobility in anaerobic pore waters during bacterial decomposition of organic matter. Initial preservation of organic matter was mediated by high sedimentation rates (36 m/m.y.). High iron (Fe) and manganese (Mn) contents are associated with carbonate concretions of siderite, manganosiderite, and rhodochrosite. These concretions probably formed in response to elevated pore-water alkalinity and total dissolved carbon dioxide (CO2) concentrations resulting from bacterial sulfate reduction, as indicated by nodule stable-isotope compositions and pore-water geochemistry. These nodules differ from those found in upper Cenozoic hemipelagic sequences in that they are not associated with methanogenesis. Phosphate minerals (carbonate-fluorapatite) precipitated in some intervals, probably as the result of desorption of phosphorus from iron and manganese during reduction.
The bulk chemical composition of the sediments differs little from that of North Atlantic Quaternary abyssal red clays, but may contain a minor hydrothermal component. The silicon/ aluminum (Si/ Al) ratio, however, is high and variable and probably reflects original variations in biogenic opal, much of which is now altered to smectite and/or opal CT. An increase in the sodium/potassium (Na/K) ratio in the upper Eocene corresponds to the beginning of coarser-grained feldspar flux to the site, possibly marking the onset of more vigorous deep currents.
Although the Site 647 cores provide a nearly complete high-resolution, high latitude Eocene-Oligocene record, the high sedimentation rate and somewhat unusual diagenetic conditions have led to variable alteration of benthic foraminifers and fine fraction carbonate and have overprinted the original stable-isotope records. Planktonic foraminifers are less altered, but on the whole, there is little chance of sorting out the nature and timing of environmental change on the basis of our stable-isotope analyses.
A total of 117 samples of predominantly grayish-green, middle Eocene through lower Oligocene, nannofossil claystone and clayey nannofossil chalk from lithologic Unit IIIC (260-518 mbsf; Cores 105-647A-28R through -54R) in Hole 647A were collected for inorganic geochemical and stable isotopic analyses. The samples were collected initially for the following purposes:
- To document possible changes in the geochemical composition of sediments and stable isotopic compositions of calcareous biogenic components in response to changes in paleoclimatic and ocean circulation during the late Eocene to early Oligocene at high northern latitudes.
- To document the composition of unusual authigenic nodules and/or beds in lithologic Unit IIIC.
The generally good core recovery, an apparently complete Eocene/Oligocene transition, and seemingly good (visual) preservation of calcareous microfossils led us to believe that our results would have a bearing on interpretation of climate and circulation events that occurred in latest Eocene to earliest Oligocene time. However, as shown below, the most significant application for our geochemical and isotopic data is in the documentation of a rather unusual early diagenetic regime.
Samples were freeze-dried and ground to pass through a 100-mesh (149 µm) sieve. Splits of all samples were analyzed coulometrically (Huffman, 1985) for carbonate and total carbon (precision better than 1%) at the University of Rhode Island. (URI). In addition, splits of many samples were sieved and selected for planktonic and benthic foraminifers for stable isotope studies. Splits of the ground samples were analyzed at the U.S. Geological Survey (USGS), Denver, for 10 major and minor elements by X-ray fluorescence (XRF) and 30 major, minor, and trace elements by induction-coupled argon-plasma emission spectrometry (ICP) (Baedecker, 1987). Nine elements (aluminum, iron, magnesium, calcium, sodium, potassium, titanium, phosphorus, and manganese) were analyzed by both XRF and ICP, with essentially identical results between the two methods. Five samples contained concentrations of iron (Fe) and manganese (Mn) that were sufficiently high to cause interferences with XRF; hence, no XRF results are available for these samples. Analytical results are given in Table 1 and plotted vs. depth in Figures 1 through 6. The geochemical data have not been corrected for contribution of pore-water salts because porosities are typically lower than 40%. Sodium (Na) and magnesium (Mg) concentrations would be the main elements affected.
Carbonate components were analyzed for carbon and oxygen-isotopic compositions at URI. Samples were ground to <50 µm, roasted in vacuo at 390˚C for 1 hr and reacted online in purified phosphoric acid at 50˚C. The resulting CO2 gas was purified and introduced into a VG Micromass 602D mass spectrometer for analysis.