ALBERT

Description: Benthic oxygen and carbon isotopic results from a depth transect on Maud Rise, Antarctica, provide the first evidence for Warm Saline Deep Water (WSDW) in the Paleogene oceans. Distinct reversals occur in the oxygen isotopic gradient between the shallower Hole 689B (Eocene depth ~1400 m; present-day depth 2080 m) and the deeper Hole 690B (Eocene depth ~2250 m; present-day depth 2914 m). The isotopic reversals, well developed by at least 46 Ma (middle middle Eocene), existed for much of the remaining Paleogene. We do not consider these reversals to be artifacts of differential diagenesis between the two sites or to have resulted from other potentially complicating factors. This being so, the results show that deep waters at Hole 690B were significantly warmer than deep waters at the shallower Hole 689B. A progressive decrease and eventual reversal in benthic to planktonic delta18O gradients in Hole 690B, demonstrate that the deeper waters became warmer relative to Antarctic surface waters during the Eocene. The warmer deep waters of the Paleogene are inferred to have been produced at middle to low latitudes, probably in the Tethyan region which contained extensive shallow-water platforms, ideal sites for the formation of high salinity water through evaporative processes. The ocean during the Eocene, and perhaps the Paleocene, is inferred to have been two-layered, consisting of warm, saline deep waters formed at low latitudes and overlain by cooler waters formed at high latitudes. This thermospheric ocean, dominated by halothermal circulation we name Proteus. The Neogene and modern psychrospheric ocean Oceanus is dominated by thermohaline circulation of deep waters largely formed at high latitudes. An intermediate condition existed during the Oligocene, with a three-layered ocean that consisted of cold, dense deep waters formed in the Antarctic (Proto-AABW), overlain by warm, saline deep waters from low latitudes, and in turn overlain by cool waters formed in the polar regions. This we name Proto-oceanus which combined both halothermal and thermohaline processes. The sequence of high latitude, major, climatic change inferred from the oxygen isotopic records is as follows: generally cooler earlier Paleocene; warming during the late Paleocene; climax of Cenozoic warmth during the early Eocene and continuing into the early middle Eocene; cooling mainly in a series of steps during the remainder of the Paleogene. Superimposed upon this Paleogene pattern, the Paleocene/Eocene boundary is marked by a brief but distinct warming that involved deep to surface waters and a reduction in surface to deep carbon and oxygen isotopic gradients. This event coincided with major extinctions among the deep-sea benthic foraminifers as shown by Thomas (1990 doi:10.2973/odp.proc.sr.113.123.1990). Salinity has played a major role in deep ocean circulation, and thus paleotemperatures cannot be inferred directly from the oxygen isotopic composition of Paleogene benthic foraminifers without first accounting for the salinity effect.

Description: A detailed paleomagnetic study was carried out on biosiliceous and calcareous sediments drilled on Maud Rise, Antarctica, during ODP Leg 113. High-quality APC sections were retrieved in the upper 220 m of Holes 689B and 690B. Average deposition rates range from 3 to 15 m/m.y. A close (25 cm) paleomagnetic sample spacing provided a medium-resolution magnetostratigraphic sequence for the Paleogene and Neogene. Paleomagnetic samples were demagnetized stepwise by alternating fields, and characteristic remanent magnetization directions were derived from detailed vector and difference vector component analysis. A magnetochronologic framework has been established for the first time for the Southern Ocean sedimentary sequences spanning Paleocene to Oligocene and middle Miocene to early Pliocene times. Biosiliceous and calcareous microfossil stratigraphies were used to constrain magnetostratigraphic age assignments. Although average sedimentation rates were rather low, nearly complete sections of the geomagnetic polarity time scale (e.g., Chrons C5 and C5A) could be correlated with the inferred polarity pattern. Miocene and Pliocene records are marked by a high number of hiatuses mainly identified by diatom biostratigraphy. Good paleomagnetic correlation between the two holes is afforded in particular in the middle to upper Miocene. Oligocene magnetostratigraphy reveals a high-quality paleomagnetic record with a mostly complete Oligocene section in Hole 689B at ~5 m/m.y. deposition rate. Hole 690B exhibits higher deposition rates (7-12 m/m.y.), although two hiatuses are present. Early and late Eocene sedimentary sequences could be analyzed in both holes, but in Hole 689B middle Eocene chrons were disrupted by hiatuses and only incomplete polarity intervals C21 and C24 were encountered. Highest resolution (14 m/m.y.) was achieved in Hole 690B in a complete early Eocene and late Paleocene sequence from Chrons C23 to C26, with a number of short polarity intervals detected within Chrons C24 and C25.

Description: Middle Eocene to Late Oligocene sediments from near the crest (Site 689B, water depth 2080 m) and flank (water depth 2914 m) of the Maud Rise (62°S) have been investigated by coarse fraction analysis and have revealed the following: (1) The middle Eocene (50-40 Ma) was a period of pure carbonate sedimentation, with good preservation of carbonate microfossils. No opal 〉 40 µm is present. (2) In the late Eocene (40-36.5 Ma) opal fossils (mainly radiolaria, and some diatoms 〉 40 µm) appeared for the first time. Three maxima in opal sedimentation (Eocene/Oligocene boundary, middle early Oligocene and early/late Oligocene boundary) are separated by increases in carbonate sedimentation. The dissolution of carbonate fossils is strong in the opal-rich layers. Opal sedimentation is attributed to cooling and probably more vigorous atmospheric circulation and increased upwelling. (3) Carbonate dissolution increased with water depth in the Oligocene, whereas in the middle Eocene excellent carbonate preservation in the deeper Site 690B and stronger dissolution in the shallower Site 689B is attributed to different bottom-water characteristics. The middle Eocene bottom water probably was formed by strong evaporation at low latitudes, whereas by the earliest Oligocene formation of Antarctic Bottom Water (AABW) had set in. (4) Current influence, not on top but on the flank of the Maud Rise, could be recorded by means of larger grain sizes of benthonic and planktonic microfossils. (5) Ice-rafted debris was not found. Quartz and other minerals are very rare and not larger than 125 µm and may have been supplied by ice as well as by wind or by deep currents. Mica contents were up to 10 times higher in the middle Eocene on the flank compared to on the crest of the Maud Rise, indicating deep current supply.

Description: At Sites 689 and 690, drilled during ODP (Ocean Drilling Program) Leg 113 on the Maud Rise (southeast Weddell Sea), moderately to well preserved radiolarian assemblages were obtained from continuously recovered upper Oligocene and Neogene sequences. Based on radiolarian investigations, a biostratigraphic zonation for a time interval covering the late Oligocene to the middle Miocene is proposed. The radiolarian zonation comprises 10 zones. Five zones are new, and five zones previously defined by Chen (1975) were modified. The zones and the ranges of the nominate species are directly calibrated with a geomagnetic polarity record. This is the first attempt at a direct correlation of late Oligocene to middle Miocene radiolarian zones with the geomagnetic time scale. Six hiatuses were delineated in the studied upper Oligocene to middle Miocene sections. One major hiatus, spanning ca. 6 m.y., is between the upper Oligocene and the lower Miocene sequences. Another important hiatus separates the lower and middle Miocene sediments. As a base for the biostratigraphic investigations, a detailed taxonomic study of the recovered radiolarian taxa is achieved. Three new radiolarian species that occur in upper Oligocene and lower Miocene sediments are described (Cycladophora antiqua, Cyrtocapsella robusta, and Velicucullus altus).

Description: Five species of Bolboforma have been found in middle Eocene to lower Oligocene sediments from Maud Rise, Weddel Sea, Antarctica (Leg 113, Holes 689B and 690B), the first reported Bolboforma from the Antarctic Paleogene. The previous oldest known occurrences of Bolboforma in the world's oceans were of late Eocene age and this study extends the known range to the middle middle Eocene (~ 44 Ma). Highest species diversity of Bolboforma in the Weddell Sea region of Antarctica occurred during the late Eocene, after which all but one important species disappeared before the Eocene/Oligocene boundary (36.5 Ma). The remaining species, B. irregularis, disappeared soon after, during the earliest Oligocene. The disappearance of Bolboforma in this region of Antarctica coincided with significant climatic cooling that occurred at the end of the Eocene and during the earliest Oligocene, when subpolar replaced temperate conditions. Bolboforma is not known from younger sediments in the Antarctic except for a brief interval during the late early Miocene, an interval of Neogene climatic warmth. The presence of Bolboforma in Eocene to lower Oligocene sequences in the Weddell Sea region of Antarctica is therefore consistent with this taxon's previously recognized association with temperate water masses. Bolboforma is of limited biostratigraphic value at present, because of relatively long stratigraphic ranges and diachronous extinctions. Previous suggestions that Bolboforma represents an encystment stage of phytoplankton require further critical study because the deposition, in large numbers, at paleodepths up to 2250 m in the open ocean, is an unlikely strategy for an encystment phase of a phytoplanktonic organism. A new species, Bolboforma antarctica, is described, exhibiting a stratigraphic range from middle middle Eocene to the upper Eocene (~ 44 to 39 Ma).