Planktonic foraminifera, nannofossils and stable isotope record across tghe Paleocene-Eocene thermal maximum of ODP Hole 113-690B

The carbonate saturation profile of the oceans shoaled markedly during a transient global warming event known as the Paleocene-Eocene thermal maximum (PETM) (circa 55 Ma). The rapid release of large quantities of carbon into the ocean-atmosphere system is believed to have triggered this intense episode of dissolution along with a negative carbon isotope excursion (CIE). The brevity (120-220 kyr) of the PETM reflects the rapid enhancement of negative feedback mechanisms within Earth's exogenic carbon cycle that served the dual function of buffering ocean pH and reducing atmospheric greenhouse gas levels. Detailed study of the PETM stratigraphy from Ocean Drilling Program Site 690 (Weddell Sea) reveals that the CIE recovery period, which postdates the CIE onset by ~80 kyr, is represented by an expanded (~2.5 m thick) interval containing a unique planktic foraminiferal assemblage strongly diluted by coccolithophore carbonate. Collectively, the micropaleontological and sedimentological changes preserved within the CIE recovery interval reflect a transient state when ocean-atmosphere chemistry fostered prolific coccolithophore blooms that suppressed the local lysocline to relatively deeper depths. A prominent peak in the abundance of the clay mineral kaolinite is associated with the CIE recovery interval, indicating that continental weathering/runoff intensified at this time as well (Robert and Kennett, 1994). Such parallel stratigraphic changes are generally consonant with the hypothesis that enhanced continental weathering/runoff and carbonate precipitation helped sequester carbon during the PETM recovery period (e.g., Dickens et al., 1997, doi:10.1130/0091-7613(1997)025<0259:ABOGIT>2.3.CO;2 ; Zachos et al., 2005, doi:10.1126/science.1109004).

Cenozoic magnetostratigraphy of ODP holes 113-689B and 113-690B

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.

Magnetic susceptibility, XRF and color reflectance data of DSDP Hole 72-516F, cores 113 and 114

Deep marine successions of early Campanian age from DSDP site 516F drilled at low paleolatitudes in the South Atlantic reveal distinct sub-Milankovitch variability in addition to precession and eccentricity related variations. Elemental abundance ratios point to a similar 5 climatic origin for these variations and exclude a quadripartite structure - as observed in the Mediterranean Neogene - of the precession related cycles as an explanation for the inferred semi-precession cyclicity in MS. However, the semi-precession cycle itself is likely an artifact, reflecting the first harmonic of the precession signal. The sub-Milankovitch variability is best approximated by a ~ 7 kyr cycle as shown by 10 spectral analysis and bandpass filtering. The presence of sub-Milankovitch cycles with a period similar to that of Heinrich events of the last glacial cycle is consistent with linking the latter to low-latitude climate change caused by a non-linear response to precession induced variations in insolation between the tropics.

Analysis of an ancient global warming event, the Paleocene-Eocene thermal maximum, under examination from ODP Holes 113-689B and 113-690B

Spatiotemporal patterns of carbonate dissolution provide a critical constraint on carbon input during an ancient (~55.5 Ma) global warming event known as the Paleocene-Eocene thermal maximum (PETM), yet the magnitude of lysocline shoaling in the Southern Ocean is poorly constrained due to limited spatial coverage in the circum-Antarctic region. This shortcoming is partially addressed by comparing patterns of carbonate sedimentation at the Site 690 PETM reference section to those herein reconstructed for nearby Site 689. Biochemostratigraphic correlation of the two records reveals that the first ~36 ka of the carbon isotope excursion (CIE) signaling PETM conditions is captured by the Site 689 section, while the remainder of the CIE interval and nearly all of the CIE recovery are missing due to a coring gap. A relatively expanded stratigraphy and higher carbonate content at mid-bathyal Site 689 indicate that dissolution was less severe than at Site 690. Thus, the bathymetric transect delimited by these two PETM records indicates that the lysocline shoaled above Site 689 (~1,100 m) while the calcite compensation depth remained below Site 690 (~1,900 m) in the Weddell Sea region. The ensuing recovery of carbonate sedimentation conforms to a bathymetric trend best explained by gradual lysocline deepening as negative feedback mechanisms neutralized ocean acidification. Further, biochemostratigraphic evidence indicates the tail end of the CIE recovery interval at both sites has been truncated by a hiatus most likely related to vigorous production and advection of intermediate waters.

Geochemical investigation of ODP Leg 113 sites

Dissolved organic carbon (DOC) was determined in pore water extracted from pelagic and hemipelagic sediments recovered during Leg 113. DOC concentration varied between 1.82 and 13.6 mg C/L which is one to two orders of magnitude less than previously reported for hemipelagic sediments. It is argued that this difference is related to differences in the intensity of degradation of organic matter. As a first approximation it is found that in reducing sediments, the level of DOC is proportional to the intensity of sulfate reduction. It is suggested that DOC is formed by different mechanisms in oxic and reducing environments.

Investigations of Cretaceous and Tertiary kerogens in sediments of ODP Leg 113 holes

Seventy-one samples from nine sites were analyzed for total organic carbon (TOC). Fifty-six samples, containing 0.2% or more TOC, were evaluated by Rock-Eval to assess the nature of their kerogen and its petroleum source potential. Visual kerogen studies were carried out. Petroleum potential was encountered only in Valanginian calcareous claystones at Hole 692B close to the margin of Dronning Maud Land. A section of 44.7 m was penetrated. The unit possesses a revised mean TOC of 9.8% and petroleum potential of 43.2 kg/Mg, relatively high values in comparison to other Cretaceous anoxic oceanic sections and the totality of petroleum source rocks. At Sites 689 and 690, extremely low TOC levels, mean 0.07%, preclude kerogen analysis. Kerogens in Eocene to Pliocene sediments of the central and western Weddell Sea (Sites 694, 695, 696, and 697) are similar everywhere, largely comprising brown to black, granular, amorphous material of high rank, and generally possessing several reflectance populations of vitrinite particles. The latter are interpreted as indicative of the recycling of sediments of a variety of levels of thermal maturity.

Interstitial-water chemistry of ODP Leg 113 sites

Variations in the distribution of major elements and stable oxygen isotopes in ODP Leg 113 pore water are not related to lithology and thus appear to be controlled by minor constituents. Petrographic observations and geochemical considerations indicate that alteration of calc-alkalic volcanic material dispersed in the sediment is an important process. A diagenetic reaction is constructed that involves transformation of volcanic glass into smectite, zeolite (represented by phillipsite), chert, and iron sulfide. Mass balance calculations reveal that alteration of less than 10% (volume) of volcanogenic material may account for the observed depletion of magnesium, potassium, and 18O and enrichment of calcium. Alteration of this amount of volcanic glass produces less than 4% (volume) of smectite and zeolite. Hence, mass balance is obtained without having to invoke unreasonable large amounts of volcanic matter or interactions between seawater and basement.

Lower Cretaceous nannofossils of ODP Holes 113-682B and 113-693A

Organic-rich, moderately to sparsely nannofossiliferous Lower Cretaceous claystones ("black shales") were cored at two Ocean Drilling Program Leg 113 sites on the continental slope of East Antarctica off Dronning Maud Land. A 39 m section at Site 692 yielded a Neocomian assemblage of limited diversity with rare Cyclagelosphaera deflandrei, Diadorhombus rectus, and Cruciellipsis cuvillieri, and is probably Valanginian in age. A 70-m section at Site 693 is assigned to the Rhagodiscus angustus Zone (late Aptian-early Albian in age). The latter zone is represented at DSDP sites on the Falkland Plateau, but equivalents to the Neocomian section are absent there, probably due to a disconformity. Watznaueria barnesae is the dominant species at both ODP sites, but it shares dominance with Repagulum parvidentatum at Site 693, where they total 70%-90% of the assemblage; their dominance is attributed to a paleogeographic setting within a restricted basin rather than to postdepositional dissolution of other species. The evolutionary development of this restricted basin and its eventual ventilation in early Albian times is discussed in terms of the regional stratigraphy and the breakup and dispersal of southwestern Gondwanaland. One new species, Corollithion covingtonii, is described.

Isotopic composition and Strontium/Calcium ratios of foraminifera of ODP Holes 113-689B and 113-690C

Oxygen and carbon isotopic ratios were measured from Maestrichtian benthic and planktonic foraminifer species and bulk carbonate samples from ODP Sites 689 and 690, drilled on the Maud Rise during Leg 113. Careful scanning electron microscope observations reveal that test calcite in some intervals was diagenetically altered, although Sr/Ca and isotopic ratios of these tests do not appear to have been modified significantly. Foraminifer d18O values at both sites document a cooling trend during early Maestrichtian time, a rapid drop in water temperatures at the time of the first appearance of Abathomphalus mayaroensis in the high southern latitude regions (about 69.9 Ma), and lower water temperatures during late Maestrichtian time. d13C values record a depletion in 13C in the latest early Maestrichtian time beginning at about 72.2 Ma, just prior to the sharp late Maestrichtian increase in d18O values. These trends are similar to those previously reported for well-preserved benthic foraminifer species from Seymour Island, in the Antarctic Peninsula. Paleotemperature estimates are also comparable to those at Seymour Island and suggest temperate climatic conditions in Antarctica and that bottom waters in the southern South Atlantic region were of Antarctic origin. Benthic and planktonic foraminifer 613C values fluctuate sympathetically and are higher in upper Maestrichtian sediments than in the lower Maestrichtian sequence.