Eocene siliceous and calcareous phytoplankton, Deep Sea Drilling Project Holes 95-612 and 95-613

Eocene siliceous and calcareous phytoplankton, with emphasis on silicoflagellates, were studied in 62 samples from DSDP Sites 612 and 613 on the continental slope and rise off New Jersey. The mid-latitude assemblages correlate well with assemblages from California, Peru, and offshore of southern Brazil, but are distinctly different from high-latitude cold-water assemblages of the Falkland Plateau off southern Argentina. Coccoliths and silicoflagellates provide evidence for the presence of a fairly complete middle and upper Eocene sequence, represented by a composite of Sites 612 and 613. A major unconformity occurs at the middle Eocene to upper Eocene contact at Site 612. The genus Bachmannocena Locker is emended and proposed as a replacement for genus Mesocena Ehrenberg for ring silicoflagellates. Six new silicoflagellates and one new diatom are described: Bachmannocena apiculata monolineata Bukry, n. subsp., Corbisema amicula Bukry, n. sp., C. bimucronata elegans Bukry, n. subsp., C. hastata incohata Bukry, n. subsp., C. jerseyensis Bukry, n. sp., Dictyocha acuta Bukry, n. sp., and Coscinodiscus eomonoculus Bukry, n. sp. Also, one new replacement name, B. paulschulzn Bukry, nom. nov., and 24 new combinations are proposed for genus Bachmannocena.

Late Albian foraminiferal carbon and oxygen isotope data and biostratigraphic datums from ODP holes 171-1050C and 171-1052E

A late Albian-early Cenomanian record (~103.3 to 99.0 Ma), including organic-rich deposits and a d13C increase associated with oceanic anoxic event 1d (OAE 1d), is described from Ocean Drilling Program sites 1050 and 1052 in the subtropical Atlantic. Foraminifera are well preserved at these sites. Paleotemperatures estimated from benthic d18O values average ~14°C for middle bathyal Site 1050 and ~17°C for upper bathyal Site 1052, whereas surface temperatures are estimated to have ranged from 26°C to 31°C at both sites. Among planktonic foraminifera, there is a steady balance of speciation and extinction with no discrete time of major faunal turnover. OAE 1d is recognized on the basis of a 1.2 per mill d13C increase (~100.0-99.6 Ma), which is similar in age and magnitude to d13C excursions documented in the North Atlantic and western Tethys. Organic-rich "black shales" are present throughout the studied interval at both sites. However, deposition of individual black shale beds was not synchronous between sites, and most of the black shale was deposited before the OAE 1d d13C increase. A similar pattern is observed at the other sites where OAE 1d has been recognized indicating that the site(s) of excess organic carbon burial that could have caused the d13C increase has (have) yet to be found. Our findings add weight to the view that OAEs should be chemostratigraphically (d13C) rather than lithostratigraphically defined.

Radiolarian chronology and fauna across the middle/late Eocene boundary at ODP Hole 171-1052A

Quantitative radiolarian assemblage analysis has been conducted on middle and upper Eocene sediments (Zones RP16 to RP18) from Ocean Drilling Program Site 1052 in order to establish the radiolarian magnetobiochronology and determine the nature of the faunal turnover across the middle/late Eocene boundary in the western North Atlantic Ocean. We recognize and calibrate forty-five radiolarian bioevents to the magneto- and cyclo-stratigraphy from Site 1052 to enhance the biochronologic resolution for the middle and late Eocene. Our data is compared to sites in the equatorial Pacific (Leg 199) to access the diachrony of biostratigraphic events. Eleven bioevents are good biostratigraphic markers for tropical/subtropical locations (south of 30°N). The primary markers (lowest occurrences of Cryptocarpium azyx and Calocyclas bandyca) which are tropical zonal boundary markers for Zones RP17 and RP18 provide robust biohorizons for correlation and age determination from the low to middle latitudes and between the Atlantic and Pacific Oceans. Some other radiolarian bioevents are highly diachronous (<1 million years) between oceanic basins. A significant faunal turnover of radiolarians is recognized within Chron C17n.3n (37.7 Ma) where 13 radiolarian species disappear rapidly in less than 100 kyr and 4 new species originate. The radiolarian faunal turnover coincides with a major extinction in planktonic foraminifera. We name the turnover phase, the Middle/Late Eocene Turnover (MLET). Assemblage analysis reveals the MLET to be associated with a decrease in low-mid latitude taxa and increase in cosmopolitan taxa and radiolarian accumulation rates. The MLET might be related to increased biological productivity rather than to surface-water cooling.

Quantity, provenance, and thermal maturity of organic matter at DSDP Holes 95-612 and 95-613

Using methods of analysis from organic geochemistry and organic petrography, we investigated six Pliocene to Maestrichtian samples from DSDP Site 612 and five Pliocene to Eocene samples from DSDP Site 613 for the quantity, type, and thermal maturity of organic matter. At both sites, organic carbon content is low in the Eocene samples (0.10 to 0.20%) and relatively high in the Pliocene/Miocene samples (0.87 to 1.15%). The Maestrichtian samples from Site 612 contain about 0.6% organic carbon. The organic matter is predominantly terrigenous, as indicated by low hydrogen index values from Rock-Eval pyrolysis and the dominance of long-chain wax alkanes in the extractable hydrocarbons. The organic matter is at a low level of thermal maturity; measured vitrinite reflectance values were between 0.27 and 0.44%.

Planktic foraminiferal sieve size specific d13C and d18O values and maximum test diameter from ODP sites 171-1051 and 120-748

Many genera of modern planktic foraminifera are adapted to nutrient-poor (oligotrophic) surface waters by hosting photosynthetic symbionts, but it is unknown how they will respond to future changes in ocean temperature and acidity. Here we show that ca. 40 Ma, some fossil photosymbiont-bearing planktic foraminifera were temporarily 'bleached' of their symbionts coincident with transient global warming during the Middle Eocene Climatic Optimum (MECO). At Ocean Drilling Program (ODP) Sites 748 and 1051 (Southern Ocean and mid-latitude North Atlantic, respectively), the typically positive relationship between the size of photosymbiont-bearing planktic foraminifer tests and their carbon isotope ratios (d13C) was temporarily reduced for ~100 k.y. during the peak of the MECO. At the same time, the typically photosymbiont-bearing planktic foraminifera Acarinina suffered transient reductions in test size and relative abundance, indicating ecological stress. The coincidence of minimum d18O values and reduction in test size-d13C gradients suggests a link between increased sea-surface temperatures and bleaching during the MECO, although changes in pH and nutrient availability may also have played a role. Our findings show that host-photosymbiont interactions are not constant through geological time, with implications for both the evolution of trophic strategies in marine plankton and the reliability of geochemical proxy records generated from symbiont-bearing planktic foraminifera.

Eocene calcareous nannofossils in DSDP Holes 95-612 and 95-613

Lower Eocene calcareous nannofossil limestone cored at DSDP Site 612 on the middle slope off New Jersey represents an almost complete biostratigraphic sequence; only the lowest biozone (CP9a; NP10*) was not recovered. The thickness of the strata (198 m), the good preservation of the nannofossils, and the lack of long hiatuses justify the acceptance of this section as a lower Eocene reference for the western North Atlantic margin. The widely recognized and very similar nannofossil zonations of Martini (NP zones) and Bukry-Okada (CP zones) are emended slightly to make their lower Eocene biozones coeval; in addition, five new subzones are erected that subdivide zones CP10 and CPU (NP12 and NP13). Established biozone names are retained as they are altered little in concept, but alphanumeric code systems are changed somewhat by appending an asterisk (*) to identify zones that are emended. Zone CP10* (NP12*) is divided into two parts, the Lophodolithus nascens Subzone (CP10*a; NP12*a) and the Helicosphaera seminulum Subzone (CP10*b; NP12*b). Zone CPU* (NP13*) is divided into three parts, the Helicosphaera lophota Subzone (CP11*a; NP13*a), the Cyclicargolithuspseudogammation Subzone (CP11*b; NP13*b), and the Rhabdosphaera tenuis Subzone (CP11*c; NP13*c). At Site 612, a time-depth curve based on nannofossil datums dated in previous studies reveals a smoothly declining sediment accumulation rate, from 4.9 cm/10**3yr in CP10* (NP12*) to 2.8 cm/103 yr. in CP12* (NP14*). The ages of first-occurrence datums not previously dated are approximated by projection onto this timedepth curve and are as follows: Helicosphaera seminulum, 55.0 Ma; Helicosphaera lophota, 54.5 Ma; Cyclicargolithus pseudogammation, 53.7 Ma; Rhabdosphaera tenuis, 52.6 Ma; and Rhabdosphaera inflata, 50.2 Ma. At nearby Site 613 on the upper rise, strata of similar age, 139 m thick, contain an unconformity representing Subzone CPll*b (NP13*b) and a hiatus of approximately 1.1 m.y. duration. The sediment accumulation rate in the lower part of this section (9.7 cm/10**3yr.) is twice that observed for equivalent strata at Site 612. The hiatus and the heightened sediment accumulation rate at Site 613 probably represent the effects of episodic mass wasting on the early Eocene continental slope and rise.