DEPOSITION AND DIAGENETIC ALTERATION OF SEDIMENT IN EMERALD BASIN, THE SCOTIAN SHELF

Two sets of sediment cores taken from the margin and centre of Emerald Basin on the Scotian Shelf penetrated 6.3 and 1.074 m of silty clay. These sediments were deposited since Wisconsinan deglaciation. The lowermost sediments were deposited during about 14,000-17,000 years ago, when glacial ice occupied parts of the continental shelf. These silty sediments contained modest amounts of organic matter deposited under brackish saline conditions. The next younger depositional sequence was glaciomarine and was deposited when glacial ice had retreated from most of the shelf. These sediments are overlain by a marine transitional sequence deposited during the time of rising sea level. During this time an increased amount of organic matter was deposited and preserved (up to 2.2%) at the deposition site in the centre of the basin. These central Basin sediments accumulated at about 0.4 m per 1000 years which is about twice the rate of the marginal deposits. The most recent sediments represent present-day deposits derived mainly from winnowing of sandy and silty banks surrounding Emerald Basin. A distinct difference between the marginal deposits and the central basin deposits has evolved over time with the modern central Basin deposits being finer, more organic-rich and containing about 10% CaCO3 as compared with about 5% CaCO3 in the reworked marginal sediments. A distinct imprint of diagenetic processes has been retained in the sediments and contained pore water. Four diagenetic zones have been defined on the basis of sulphate and ammonia concentrations in the pore water. These zones are characterized by sulphate reduction and ammonia production with depth in the sediments. The depletion of sulphate to undetectable levels and the decrease in pE to less than 3.0 (-log M) at about 10 m depth in the sediments from the centre of the Basin has resulted in precipitation of about 0.5% hydrated iron sulphides, and poising of the sediments for anoxic production of methane.