Carbonate Dissolution Facies of Late Miocene to Pleistocene Sediments from Leg 73, Sites 519 and 520 (South Atlantic)

Abstract

The dissolution fades of 68 sediment samples from Leg 73 were determined by analyzing carbonate content and grain size. All sediments (Miocene to Pleistocene) from Site 519 and the Pliocene sediments from Site 520 were eolytic or oligolytic; that is, they were deposited above the lysocline. Miocene sediments from Site 520 were mesolytic or pleistolytic (deposited below the lysocline). INTRODUCTIONHsü and Andrews established five dissolution facies for pelagic sediments (1970). This definition was slightly modified by Violanti et al. (1979), a modification that resulted in the following scheme: In other words, the dissolution facies of a sediment can be defined by determining the content of terrigenous elastics (or the CaCO 3 content) and the percentage of foraminifers (i.e., the size of the fraction >63µm.) This work was done on sediment samples from Leg 73, Sites 519 and 520, and the results are reported herein. Other parameters that could define a dissolution facies, like the ratio between planktonic and benthic foraminifers, the diversity of the planktonic fauna, and the number of whole tests per gram of fraction > lOOµm (e.g., Violanti et al., 1979;Thunell, 1976), have not been investigated. METHODOLOGYSixty-one samples from Site 519 and eight samples from Site 520 were analyzed. The sub-bottom depths, lithologic units, and ages of the samples are listed in Table 1. About 5 to 10 g of each sample were freeze dried. One part of each dried sample, about 1 g, was used for the determination of CaCO 3 content. The analysis was done with a Ströhlein Coulomat. The sediment is burned in a O 2 current, the resulting CO 2 is volumetrically analyzed, and the total carbon content of the sample is printed out. By operating the Coulomat with N 2 , only the CO 2 from the carbonates is measured, because the organic carbon is not oxidized as it is in the O 2 current. The difference between the two measurements is equal to the organic carbon content. The sediments of Sites 519 and 520 do not contain a significant amount of organic carbon.For our purposes, the carbon contents (measured with N 2 ) of the samples were multiplied by the stoichiometric factor 8.33 to obtain the CaCO 3 percentage (see Table 1). The percentage of terrigenous matter is 100% minus the percentage of CaCO 3 .The second part of the dried sample was used for the grain-size analysis. About 3 to 6 g of each sample were weighed exactly and soaked with H 2 O 2 . After 30 min. the samples were placed in an ultrasonic bath for 30 s. Distilled water was added and the samples were wet sieved through 180µm, 90µm, and 63µm sieves and a filter. The sieve fractions were dried and weighed.Calculations were made to acquire the weight percentages of the fractions >180µm, 90 to 180µm, >90µm, 63 to 90/un, >63µm, and the proportions >90µm/>63µm and > 180µm/>90/un (see Table 1). Graphical representations of the results are shown in Figures 1,2, and 3.