RESPONSE OF BENTHIC FORAMINIFERA TO EARLY CENOZOIC PALEOCEANOGRAPHIC CHANGES IN THE SOUTHERN OCEAN

Abstract

The Kerguelen Plateau hosts a long and continuous (late Cretaceous to Recent) archive of marine sediments which can be used to examine the nature and sequence of oceanographic events during the early Cenozoic era when the Earth was transforming from its warm house state to icehouse state. Benthic foraminiferal assemblages at ODP Hole 1138A at Central Kerguelen Plateau in the Southern Ocean manifest a combination of paleoclimatic and paleoceanographic changes which are responsible for the major faunal turnovers during Paleocene and Eocene. The present research is the first record of benthic foraminifers at this site. A total of 218 core samples of 10 cc volume spanning a depth of ~147.1 mbsf were used for the present investigation. The investigated samples cover a time span of ~ 31 Ma from early Paleocene to late Eocene. All the core samples were processed through the wet sieving method over 125 μm Tyler sieves using the common micropaleontological technique to separate the benthic foraminifera from the matrix. From each sample, ~300 benthic foraminifers were picked and mounted on the assemblage slides. A detailed systematic taxonomic study was carried out to identify 151 benthic foraminiferal species representing 56 genera. Recovered benthic foraminifers have been used for biozonation and to calculate relative abundances and species diversity. A Q-mode factor analysis was also performed over the reduced benthic foraminiferal data set in order to deduce the faunal assemblages. Benthic foraminiferal biozonation of the studied site was proposed by identifying four distinct bio-zones based on the significant zonal markers. These zones were further correlated with the calcareous nannofossil zones (CP) of Arney and Wise (2003), and bathyal benthic foraminiferal zones (BB) of Berggeren and Miller (1989) and numerical ages were updated to GTS 2012. The proposed benthic foraminiferal zones are: (i) Stensioena beccariformis-Cibicides hyphalus Interval Zone extending from an undefined base to ~56.87 Ma (CP1-CP 8 and within BB1); (ii) Aragonia aragonensis Partial Range Zone from ~56.87 to 56 Ma (CP 8 (upper part) and within BB2 (lower part); (iii) Cibicides subspiratus Interval Zone from 56 Ma to 40.5 Ma (from CP 8-CP 13 and BB2-BB3); (iv) Nuttalides truempyi Partial Range Zone from 40.5 Ma to 37.04 Ma (from CP 13-CP15 and BB4). ii Based on the faunal compositions at different time intervals, recovered benthic foraminiferal assemblages were classified into four major groups namely, Pre-Extinction fauna (~65.49 to 56.87 Ma) characterized by the well-known “Velasco-type” fauna; Extinction fauna (~56.83 to 56 Ma) composed of opportunists, stress-tolerant and survival fauna; Recovery fauna (~56 Ma to 40.5 Ma) characterized by the long-ranging fauna and Post-Recovery fauna (~40 Ma onwards) more similar to the modern forms. The Q-mode factor analysis grouped the data into the most similar or closely related assemblages at a particular interval, e.g. Stensioeina beccariformis (Sb), Cibicides cf. pseudoperlucidus (Ccp), Alabamina midwayensis (Am), Cibicides subspiratus (Cs), Nuttalides truempyi (Nt), Nuttalides umbonifer (Nu) and Oridorsalis umbonatus (Ou). These assemblages highlight the significant differences in their paleoecological preferences, for instance, Stensioena beccariformis and Cibicides cf. pseudoperlucidus assemblages of Paleocene were allied to the well oxygenated bottom water with high food resources, Alabamina midwayensis, Cibicides subspiratus and Nuttalides truempyi which dominated the assemblages of early to late middle Eocene were allied to a relatively warm oligotrophic bottom water conditions, while the late Eocene N. umbonifer and O. umbonatus assemblages suggest the cold, carbonate corrosive deep water mass and oligo-mesotrophic conditions. The present study reports the extinction of Stensioeina beccariformis assemblage during the latest Paleocene which is coeval with the well-known “Benthic Foraminiferal Extinction Event (BEE)” linked with the “Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma)”. A change in the source region of deep water formation and the onset of Warm Saline Deep Water (WSDW) appears to be the probable reason for the change in faunal composition at this site. Nevertheless, the effects of gradual cooling at high latitudes during the middle Eocene were more pronounced at this site due to its location in higher southern latitudes. This is evident from the almost abrupt extinction of the warm water tolerant species i.e., N. truempyi during middle Eocene and subsequent replacement by cold, carbonate corrosive water tolerant N. umbonifer in the late Eocene. This instability in the deep sea environment was also corroborated with the relative faunal abundances and several oscillations in values of faunal diversity indices. Faunal diversity values were recognized at different intervals of the studied section which indicated the global warmth of Paleocene at ~56 Ma; gradual cooling of high latitudes at ~44.7 Ma and 42 Ma; a short, rapid warming episode of Middle Eocene Climatic Optimum (MECO) at ~40 Ma iii succeeded by the intensification of Antarctic glaciation (~37 Ma) in late Eocene. Eocene benthic foraminiferal changes mainly correspond to the fluctuations in bottom water conditions due to the continuous cooling of bottom water. Thus, based on the variations in characteristics of present faunal data (Q-mode, relative abundance, and species diversity), three broad characteristic water masses were inferred during Paleocene and Eocene as follows: a) Well-ventilated and high food resources water: S. beccariformis dominated Paleocene assemblage indicating the presence of well-oxygenated water mass with a high food supply. Dominant cosmopolitan faunal occurrences in Paleocene assemblage support an unstratified or homogenous water mass for global oceans during Paleocene. b) Warm and low food resources water: N. truempyi and C. subspiratus dominated assemblages reflect a warm water and low food resources conditions. Such conditions prevailed during the temporary arrival of low latitude sourced WSDW at the bathyal sites from early to middle Eocene. Faunal compositions favor the presence of a sluggish and “Halothermal deep ocean” circulation and stratified water mass for Eocene interval. c) Cold, carbonate corrosive type deep water: Nuttallides umboniferus dominated assemblage confirm the presence of cold, carbonate corrosive deep water mass (proto-AABW) at the high latitudes due to the opening of seaways, intensive cooling and initiation of Antarctic glaciation during the late Eocene (~37 Ma onwards).