Studies in living and fossil foraminifers from seasonally productive regions

Abstract

Paleoecological and paleoenvironmental analysis of benthic and planktonic foraminiferal associations from sediments from the northern Adriatic Sea, the Kau Basin (Halmahera) and the Molucca Sea shed new light on the way in which various environmental factors influence foraminiferal distribution patterns. The qualitative and quantitative analyses on the faunas from these areas are presented and the results discussed in eight separate papers, compiled as chapters 1 - 8 of this thesis. In a series of three papers on the seasonally productive northern Adriatic Sea (chapters 2-4), the influence of oxygenation and food supply on the size and composition of benthic foraminiferal communities is emphasized. The paper on benthic foraminiferal microhabitat selection (chapter 2) shows that three categories of species can be distinguished on the basis of their vertical distribution patterns in the sediment. These categories include epifaunal, predominantly infaunal and potentially infaunal groups of taxa; the habitat selection appears to be closely related to the degree of oxygenation of the sea-bottom environment. From the distribution of living benthic foraminifera in fourteen sample stations in the northern Adriatic Sea (chapter 3) it is concluded that the downward organic flux is the most important factor determining the overall distribution patterns. This flux controls the complex relation between food and oxygen availability in the benthic environment. Chapter 4 deals with a study on the benthic foraminiferal record documenting the eutrophication history of the northern Adriatic Sea during the last 160 years. Three time-successive changes in benthic foraminiferal assemblages can be attributed to the impact of man-induced changes in sedimentation rates, food and oxygen availability in front of the Po delta. Anoxic, pollution-related events occurred with increasing frequency since 1960, whereas the effects these events had became even more from about 1980 onwards. The two papers on the Kau Basin, Halmahera (chapters 5 and 6) give a detailed account of its environmental evolution since the latest Pleistocene. Kau Basin, which was a freshwater lake during the last glacial maximum, became reconnected with the open ocean about 10 ka SP, due to the sea level rise at the Pleistocene - Holocene transition. Dysoxic bottom conditions prevailed throughout the Holocene. The surface waters were homothermal over the last 8 ka, as evidenced by the composition of the low-diversity planktonic associations. Changes in surface water productivity were most probably related to variations in river discharge. Palynological, planktonic foraminiferal and stable isotope data from a piston core off Halmahera are used to reconstruct glacial! interglacial contrasts in the northern Molucca Sea (chapter 7). The inferred environmental changes are interpreted in terms of changes in monsoonal circulation, lapse rate, and water exchange between the Pacific and Indian Oceans. It is concluded that the glacial climate was drier than today and that surface water temperatures were between oand 2.5° C lower than at present. In chapter 8, where benthic foraminiferal assemblages from food- enriched areas are discussed, fluctuations in relative numbers of groups of opportunistic inand epifauna from the Holocene of the Kau Basin are explained by applying the microhabitat model established for the northern Adriatic Sea. It is shown that the temporal and spatial relationships between the benthic faunas mainly mirror the effects of changes in oxygenation of the bottom environments, which, in turn, can be related to the Holocene climatic evolution