Late Pliocene carbonate mineralogy of ODP Hole 133-818B

Shedding of shallow carbonate material toward the deep slopes and basin floors is clearly tied to the position of the carbonate bank tops relative to the photic zone. The onset of bank shedding in periplatform sediments can record either the flooding of the bank tops within the photic zone during a rise in sea level following a period of exposure, referred to in the literature as the "highstand shedding" scenario, or the reentry of the bank tops into the photic zone during a lowering of sea level following a period of drowning, referred to as the "lowstand shedding" scenario. Results from Leg 133 post-cruise research on the Pliocene sequences, drilled in six sites within different slope settings of the Queensland Plateau, seem to point out that the latter "lowstand shedding" scenario can be applied to this particular carbonate system. At the Queensland Plateau sites, the early Pliocene (5.2-3.5 Ma) and the earliest part of the late Pliocene (3.5-2.9 Ma) age sequences were characterized, especially in the ôdeepö Sites 811 and 817, by pelagic sediments (foraminifers and coccoliths) and by typically pelagic sedimentation rates not exceeding 20 mm/k.y. The earliest part of the late Pliocene age section was characterized by well-developed hardgrounds in the "shallow" Sites 812 and 814 and by normal pelagic sediments mixed with reworked phosphatized planktonic foraminifers in Site 813. Finally, the early part of the late Pliocene (2.9-2.4 Ma) section was characterized by high sedimentation rates, related to the shedding and admixture into the pelagic sediments of bank-derived materials. These bank-derived materials consist of either diagenetically unaltered fine aragonite with traces of dolomite in Site 818 or micritic calcite resulting from seafloor and/or shallow burial alteration in the deepest Sites 817 and 811. The highest sedimentation rates (163 mm/k.y.) were recorded in Site 818, drilled nearest the modern carbonate bank of Tregrosse Reef. The sedimentation rates decrease with increasing distance from Tregrosse Reef - 120 mm/k.y. in Site 817 and 47.5 mm/k.y. in Site 811. The initial appearance of fine aragonite in Site 818, corresponding to the transition from pelagic to periplatform sedimentation rates, has been dated at 2.9 Ma. This Pliocene sediment pattern on the Queensland Plateau is different from the pattern observed in sediments from two earlier ODP legs (i.e., Leg 101 in the Bahamas and in Leg 115 in the Maldives), where aragonite-rich sediments, characterized by high periplatform sedimentation rates, were observed in the lower Pliocene section (5.2-3.5 Ma), whereas the upper Pliocene (3.5-1.6 Ma) sediments are more pelagic in nature and are characterized by low sedimentation rates or major hiatuses. These Pliocene periplatform sequences in the Bahamas and in the Maldives and late Quaternary age periplatform sequences worldwide have pointed out that "highstand shedding" was the typical response of carbonate platforms to fluctuations in sea level, just opposite to a "lowstand shedding" response to sea-level fluctuations, typical of siliciclastic shelves. Assuming that the envelope of Haq et al.'s (1987) sea-level curve, showing a well-defined lowering of sea level between 3.5 and 2.9 Ma, can also be applied to the southwest Pacific Ocean, based on a high-resolution Pliocene d18O record from the Ontong Java Plateau recently published by Jansen et al. (1993, doi:10.2973/odp.proc.sr.130.028.1993), the Pliocene periplatform sequences on the Queensland Plateau would have recorded the reentry of the bank tops into the photic zone during a general lowering of sea level, following an interval characterized by high sea level, during which the shallow carbonate system on the Queensland Plateau was drowned. The early Pliocene age (5.2-3.5 Ma) sediments deposited on the Queensland Plateau, an established interval of eustatic sea-level highstand, are typically pelagic in character. In addition, relatively cold surface temperatures (estimated to have ranged from 18° to 20°C by Isern et al. [this volume]) might have also stressed the reefs during early Pliocene time and contributed to the drowning of the Queensland Plateau carbonate system during the late Miocene and early Pliocene. Differential and relatively high subsidence rates, inferred by variations in paleodepth of water (based upon benthic foraminifer assemblages; Katz and Miller, this volume) may also have influenced the drowning of the carbonate bank tops on the Queensland Plateau during the late Miocene and early Pliocene. The sediments of early late Pliocene age (2.9-2.4 Ma), a well-established interval of lowering of sea level, are clearly periplatform and cyclic in nature. High-frequency (~40 k.y.) aragonite cycles, well-developed between 2.9 and 2.45 Ma, correlate with the planktonic high-resolution Pliocene d18O record from the Ontong Java Plateau, a good sea-level proxy (Jansen et al., in press). Contrary to late Quaternary age aragonite cycles from the Bahamas, the Nicaragua Rise, the Maldives, and the Queensland Plateau, the late Pliocene aragonite cycles in Hole 818B display high levels of aragonite during glacial stages and, therefore, lowstands of sea level. In addition, sediments deposited during the earliest part of the late Pliocene (3.5-2.9 Ma), transition between the early Pliocene highstand and the late Pliocene lowering in sea level, have recorded the first evidence of a fall in sea level, by (1) the occurrence of synchronous submarine hardgrounds in the two shallowest sites (Sites 812 and 814), (2) the deposition of reworked material from the shallower part of the slope into the intermediate Sites 813 and 818, and (3) the deposition of pelagic sediments in the deepest Sites 817 and 817. In summary, contrary to previous findings, the Pliocene periplatform sediments on the Queensland Plateau appear to have recorded a regional shedding of shallow carbonate bank tops during an interval of sea-level lowering, a good illustration of the "carbonate lowstand shedding" scenario, occurring during the reentry of previously drowned carbonate bank tops into the photic zone related to a decrease in sea level.

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Late Quateranry dinoflagellate cysts of ODP Site 202-1233

The late Quaternary organic-walled dinoflagellate cyst record of Site 1233 (41°S, offshore Chile) was studied with a ?200 year resolution spanning the last 25,000 years. The study provides the first continuous record of sub-recent and recent dinoflagellate cysts in the Southeast (SE) Pacific. Major changes in the composition of the cyst association, cyst concentration and morphology of Operculodinium centrocarpum reflect changes in sea surface temperature (SST), sea surface salinity (SSS), palaeoproductivity and upwelling intensity. These changes can be associated with latitudinal shifts of the circumpolar frontal systems. The high cyst concentration, high Brigantedinium spp. abundances, low species diversity and the occurrence of certain cold water species are supportive for a 7-10° equatorward shift of the Antarctic Circumpolar Current (ACC) during the coldest phase of the Last Glacial Maximum (LGM) between 25 and 21.1 cal ka BP. Deglacial warming initiated at ~18.6 cal ka BP. Termination I (18.6-11.1 cal ka BP) is interrupted by an unstable period of extreme seasonality, rather than a cooling event, between 14.4 and 13.2 cal ka BP, synchronous with the Antarctic Cold Reversal (ACR). The Holocene Maximum is observed between 11.6 and 9.8 cal ka BP and is typified by the most southward position of the northern margin of the ACC. A cooling phase occurred during the early Holocene (until ~7 cal ka BP) and during the last ~0.8 ka. Our data indicates that the SE Pacific (41°S) climate has been influenced over the whole record by changes in the Southern Hemisphere (SH) high-latitudes, while during the mid to late Holocene, also a tropical forcing mechanism was involved, including the El Niño Southern Oscillation and the variable Hadley cell intensity. Furthermore, this study showed a relationship between the variable morphology of the spines/processes of O. centrocarpum and the combined variation of sea surface salinity and temperature (SSS/SST-ratio).

Quaternary carbonate and stable isotope record of ODP Holes 133-817A and 133-818B

The Quaternary history of metastable CaCO3 input and preservation within Antarctic Intermediate Water (AAIW) was examined by studying sediments from ODP Holes 818B (745 mbsl) and 817A (1015 mbsl) drilled in the Townsville Trough on the southern slope of the Queensland Plateau. These sites lie within the core of modern AAIW, and near the aragonite saturation depth (~1000 m). Thus, they are well positioned to monitor chemical changes that may have occurred within this watermass during the past 1.6 m.y. The percent of fine aragonite content, percent of fine magnesian calcite content, and percent of whole pteropods (>355 µm) were used to separate the fine aragonite input signal from the CaCO3 preservation signal. Stable d18O and d13C isotopic ratios were determined for the planktonic foraminifer Globigerinoides sacculifer and, in Hole 818B, for the benthic foraminifer Cibicidoides spp. to establish the oxygen isotope stratigraphy and to study the relationship between intermediate and shallow water d13C of Sum CO2 and the relationship between benthic foraminiferal d13C and CaCO3 preservation within intermediate waters of the Townsville Trough. Data were converted from depth to age using oxygen isotope stratigraphy, nannostratigraphy, and foraminiferal biostratigraphy. Several long hiatuses and the absence of magnetostratigraphy did not permit time series analysis. The principal results of the CaCO3 preservation study include the following (1) a general increase in CaCO3 preservation between 0.9 and 1.6 Ma; (2) a CaCO3 dissolution maximum near 0.9 Ma, primarily expressed in the Hole 818B fine aragonite record; (3) an abrupt and permanent increase of fine aragonite content between 0.86 and 0.875 Ma in both Holes 818B and 817A probably reflecting a dramatic increase of fine carbonate sediment production on the Queensland Plateau; (4) an improvement in CaCO3 preservation near 0.87 Ma, which accompanied the increase of sediment input, indicated by the first appearance of whole pteropods in the deeper Hole 817A and a "spike" in the percent whole pteropods in Hole 818B; (5) a period of strong CaCO3 dissolution during the mid-Brunhes Chron from 0.36 to 0.41 Ma; and (6) a complex CaCO3 preservation pattern between 0.36 Ma and the present characterized by a general increase in CaCO3 preservation through time with good preservation during interglacial stages and poor preservation during glacial stages. The long-term aragonite preservation histories for Holes 818B and 817A appear to be similar in general shape, although different in detail, to CaCO3 preservation records from the deep Indian and central equatorial Pacific oceans as well as from intermediate water sites in the Bahamas and the Maldives. All of these areas have experienced CaCO3 dissolution at about 0.9 Ma and during the mid-Brunhes Chron. However, the late Quaternary (0 to 0.36 Ma) glacial to interglacial preservation pattern in Holes 818B and 817A is out of phase with CaCO3 preservation records for sediments deposited in Pacific deep and bottom waters. The sharp increase in bank production and export from the Queensland Plateau and the coincident improvement of CaCO3 preservation between 0.86 and 0.875 Ma may have been synchronous with the initiation of the Great Barrier Reef and roughly coincides with an increase in carbonate accumulation on the Bahama banks, in the western North Atlantic Ocean, and on Mururoa atoll, in the central South Pacific Ocean. The development of these reef systems during the middle Quaternary may be related to the transition in the frequency and amplitude of global sea level change from 41 k.y. low amplitude cycles prior to 0.9 Ma to 100 k.y. high amplitude cycles after 0.73 Ma. Carbon isotopic analyses show that benthic foraminiferal d13C values (Cibicidoides spp.) have been heavier than planktonic foraminiferal d13C values (G. sacculifer) throughout most of the last 0.54 m.y., which may indicate that 13C-enriched intermediate water (AAIW) occupied the Townsville Trough during much of the late Quaternary. Furthermore, both planktonic and benthic foraminiferal d13C values are often observed to be heaviest during interglacial to glacial transitions, and lightest during glacial to interglacial transitions. We suggest that this pattern is the result of changes in the preformed d13C of Sum CO2 of AAIW and may reflect changes in nutrient utilization by primary producers in Antarctic surface waters, changes in the d13C of upwelled Circumpolar Deep Water, or changes in the extent and/or temperature of equilibration between surface water and atmospheric CO2 within the Antarctic Polar Frontal Zone (the source area for AAIW). Finally, the poor correlation between percent of whole pteropods (aragonite preservation) and d13C of Cibicidoides spp. may be the result of a decoupling of d13C from CO2 due to the numerous and complex variables that combine to produce the preformed d13C of AAIW.

Cretaceous stable isotopic record and sea surface temperture estimation for ODP Site 207-1259

The Turonian (93.5 to 89.3 million years ago) was one of the warmest periods of the Phanerozoic eon, with tropical sea surface temperatures over 35°C. High-amplitude sea-level changes and positive d18O excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth. New d18O data from the tropical Atlantic show synchronous shifts ~91.2 million years ago for both the surface and deep ocean that are consistent with an approximately 200,000-year period of glaciation, with ice sheets of about half the size of the modern Antarctic ice cap. Even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice-free during past periods of intense global warming.

Nd and Pb isotope signatures of the clay-size fraction of sediment core MD99-2227

Nd and Pb isotopes were measured on the fine fraction of one sediment core drilled off southern Greenland. This work aims to reconstruct the evolution of deep circulation patterns in the North Atlantic during the Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan-African and Variscan crusts, and the Mid-Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2-6.5 ka) is composed of material derived from the North American shield and from a "young" crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the Mid-Atlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene.

Minerals and geochemistry at DSDP Hole 62-466

Alkali-basalt clasts in Upper Cretaceous sediments from Site 466 on southern Hess Rise suggest that parts of Hess Rise were constructed by off-ridge volcanic activity. Apparently, tectonic adjustments at Hess Rise occurred during the Late Cretaceous (Campanian-Maastrichtian), when parts of the original volcanic pedestal were uplifted and provided source rocks for the clasts. Synchronous volcanism may have occurred. Causes for the Late Cretaceous tectonic adjustments (and volcanism?) are not known, but they may be related to intraplate movement along the Mendocino Fracture Zone.

Hydrogen isotope data of aquatic and terrestrial lipid biomarkers from Lake Meerfelder Maar during the Younger Dryas

Here we use compound-specific hydrogen isotope data of aquatic and terrestrial lipid biomarkers from precisely dated annually laminated sediments from Lake Meerfelder Maar (MFM) in Western Germany to reconstruct decadal resolved hydroclimatic changes during the Younger Dryas. We show that cooling at MFM begun synchronous to the onset of cooling in Greenland at 12.850 years BP. Major environmental changes at MFM however took place 170 years later as a result of substantially drier conditions.

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Barium concentrations und stable isotope ratios of late Paleocene deep-sea sediments

Deep-sea sediment Ba* (Ba/Al2O3(sample) * 15% - Ba(aluminosilicate) records show increasing values synchronous with the evolution of the late Paleocene global d13C maximum, reflecting an increase in marine surface primary production and biogenic barite formation at this time. At two oligotrophic locations, Deep Sea Drilling Project (DSDP) Sites 384 and 527 in the North and South Atlantic, respectively, Ba* increases from 160-360 ppm in the early Paleocene to 1100-3000 ppm during the d13C maximum. At equatorial DSDP Site 577, positioned within or near the high-productivity zone, Ba* increases from ~15,500 ppm in the early Paleocene to ~25,400 ppm in conjunction with late Paleocene maximum d13C values. Linear fitted correlation plots of sediment Ba* content versus surface water d13C in all three regions support barite originating in the euphotic zone. The early to late Paleocene relative increase in Ba* illustrates how burial rates of Corg (relative to Al2O3) accelerated by a factor of ~1.8 and ~6.0 in the eutrophic and oligotrophic areas, respectively. A tentative estimate, weighing our result for the entire ocean, suggests that accumulation rates of organic carbon increased by a factor of 2 during the late Paleocene d13C maximum.

Foraminiferal study of ODP Hole 189-1171

Sequence boundary ages determined in shallow-water sediments obtained from ODP (Ocean Drilling Program) Leg 189 Site 1171 (South Tasman Rise) compare well with other stratigraphic records (New Jersey, United States, and northwestern Europe) and d18O increases from deep-sea records, indicating that significant (>10 m) eustatic changes occurred during the early to middle Eocene (51-42 Ma). Sequence boundaries were identified and dated using lithology, bio- and magnetostratigraphy, water-depth changes, CaCO3 content, and physical properties (e.g., photospectrometry). They are characterized by a sharp bioturbated surface, low CaCO3 content, and an abrupt increase in glauconite above the surface. Foraminiferal biofacies and planktonic/benthic foraminiferal ratios were used to estimate water-depth changes. Ages of six sequence boundaries (50.9, 49.2, 48.5-47.8, 47.1, 44.5, and 42.6 Ma) from Site 1171 correlate well to the timings of d18O increases and sequence boundaries identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and d18O increases indicates a global control and that glacioeustasy was operating in this supposedly ice-free world. This is supported by previous modeling studies and atmospheric pCO2 estimates showing that the first time pCO2 levels decreased below a threshold that would support the development of an Antarctic ice sheet occurred at ca. 51 Ma. Estimates of sea-level amplitudes range from ~20 m for the early Eocene (51-49 Ma) and ~25 m to ~45 m for the middle Eocene (48-42 Ma) using constraints established for Oligocene d18O records.

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.

Pliocene-Pleistocene biostratigraphy in the Tyrrhenian Sea

The distribution of stratigraphically important calcareous nannofossils and planktonic foraminifera has been investigated in the Pliocene-Pleistocene sequences of ODP Sites 652, 653, and 654 (Tyrrhenian Sea-western Mediterranean). Semiquantitative and quantitative methods have been used, and an optimum relative sequence of bioevents based on the calcareous plankton groups has been established. About 30 bioevents in an interval of 5.0 m.y. are considered widely traceable in the area and reasonably synchronous. On the basis of those events, a correlation among the three investigated sequences is presented.