Notes on a pyritic zone in upper Ancylus sediments from the Bothnian Sea

Abstract

Branchlike aggregates, several millimeters in length, of microscopic pyrite spherules are abundant in a narrow zone in upper Ancylus clay deposited approximately 7 500 yrs B.P. in the southeastern part of the Bothnian Sea. The aggregates in general, and often also the individual pyrite spherules, have crusts of marcasite exhibiting a radial growth pattern.In connection with stratigraphic work on Quaternary clays from the marine area west of the town of Pori, SW-Finland, small pyrite concretions were found to abound in a rather limited zone in gray homogenous clay. Samples containing concretions were taken with a modified Kullenberg piston corer. The cores, taken as far as 25 miles apart, all exhibited exactly the same sedimentary sequences.The Quaternary clays ( Fig. 1) have been deposited on till of the last glaciation. The lowermost sediments are varved clays and silts whose varve thickness decreases upwards exponentially (Ignatius, 1958). They grade into a more or less black sulphidic clay, which in turn changes rather abruptly into the gray homogenous clay containing the pyrite concretions. A few centimeters above the pyrite zone the gray clay as a rule changes sharply into a greenish gyttja-banded clay, which is overlain by a recent, dark, greenish, sulphide bearing, homogenous clay.The pyrite concretions are mainly concentrated in a layer 1-2 cm thick. No pyrite was detected in the few centimeters of gray clay separating the gyttja-banded clay from the sharp upper limit of the pyritic zone. Downwards the amount of pyritic concretions decreases gradually to vanish completely only slightly above the sulphidic clay.According to pollen and diatom analyses on cores from the Bothnian Sea the gyttja-banded clay was deposited during the marine Littorina stage of the Baltic Sea evolution (Ignatius, 1964) and corresponds to pollen zones VI-VIII. The gray clay below the gyttja-banded clay corresponds to the fresh water Ancylus stage and the upper part of zone V. Zone boundary V/VI, however, lies somewhat below the boundary between the gyttja-banded clay and the gray homogenous clay thus coinciding rather well with the main pyritic zone. The black sulphidic clay was deposited in the early part of the Ancylus stage corresponding to pollen zone IV.The age of zone-boundary V/VI and thus also the age of the pyritic layer has been estimated to be 7 500 yrs. (Kukkonen, Manuscript under preparation).The pyrite concretions exhibit variations both in size and shape (Fig. 2). The largest individuals were almost a centimeter in length but at the

Dating early Holocene palaeoseismic event(s) in the Gulf of Bothnia, Baltic Sea

“…3 the lithostratigraphy of core 27/01 is shown on the echosounding profile. Sulphide clays (53 Á68 cm) most probably represent 'lower Ancylus Lake' sediments described, for example, in Ignatius et al (1968). This is also confirmed by diatom analysis in the present study.…”

“…10/till, 20/mixed glaciofluvial sediments, 30/varved clays, 40/sulphide-bearing clays (commonly associated with the 'lower Ancylus Lake' sediments, e.g. Ignatius et al 1968), above which is a narrow layer of mainly homogeneous clay ('Upper Ancylus Lake'), 50/partly laminated gyttja clays, 60/recent gyttja/clay gyttja, G 0/gas-bearing sediment, M0/multiple echoes. Location of the profile is shown in Fig.…”

Sediment geochemistry studies in the Gulf of Finland and the Baltic Sea : a retrospective view

“…Since these pioneers many authors have reported on various characteristics and geochemistry of Baltic Sea sediments during the 1960'ies (Winterhalter 1966;Ignatius et al 1968;Papunen 1968). The investigations of the recent soft surface sediments (gyttja clays) in the Baltic Sea started in the early 1970'ies after inventions of modern study sediment samplers which provided more or less undisturbed cores with good recovery (Niemistö, Voipio 1974;Boström et al 1978;Suess 1979;Manheim 1982;Emelyanov et al 1982;Emelyanov, Kharin 1988;Boesen, Postma 1988;Jakobsen, Postma 1989;Belmans et al 1993).…”

Environmental changes during isolation processes from the Litorina Sea as reflected by diatoms and geochemical parameters -a case study

“…Ignatius et al, 1968;Winterhalter, 1992). The diatom ora is characterized by a mixture of freshwater and BF species.…”