Calcium carbonate, sedimentation and mass accumulation rates at DSDP Site 85-572

At Site 572, located at 1°N, 114° W (3903 m water depth), we recovered a continuous hydraulic piston cored section of upper Miocene to upper Pleistocene pelagic sediments. The sediment is composed of biogenic carbonate and silica with nonbiogenic material as a minor component. Detailed analysis of the calcium carbonate content shows that the degree of variability in carbonate deposition apparently changed markedly between the late Miocene and Pliocene at this equatorial Pacific site. During this interval carbonate mass accumulation rates decreased from 2.6 to 0.8 g/cm**2 per 10**3 yr. If we assume that variations in CaCO3 content reflect changes in the degree of dissolution, then the detailed carbonate analysis would suggest that the degree of variability in carbonate deposition decreases by a factor of 5 as the dominant wavelength of variations increases significantly. However, if the variability in carbonate concentration is described in terms of changes in mean mass accumulation, calculations then suggest that relatively small changes in noncarbonate rates may be important in controlling the observed carbonate records. In addition, the analysis suggests that the degree of variability observed in pelagic carbonate data may in part reflect total accumulation rates. Intervals with high sedimentation rates show lower amplitude variations in concentration than intervals with lower sedimentation rates for the same degree of change in the carbonate accumulation rate.

Thickness and duration of biostratigraphic zones, assemblages, ages and sedimentation rates at DSDP Sites 76-534 and 44-391

In this Initial Report of the Deep Sea Drilling Project, detailed studies of Sites 533 (gas hydrates) on the Blake Outer Ridge and 534 (oldest ocean history) in the Blake-Bahama Basin have provided answers to many geological and geophysical questions posed over the decade that deep drilling has been undertaken in this part of the western North Atlantic. The history of drilling and a historical review of key scientific accomplishments have been presented in the Introduction (Gradstein and Sheridan, this volume). In this final chapter we review highlights of new geological, geophysical and paleoceanographic interpretations presented in this volume, and offer a critical review of this information. We conclude with a listing of some outstanding problems and recommendations for future research, including data collection.

Calcareous nannofossil zonation, ages and sedimentation rates at DSDP Hole 76-534

A zonation is presented for the oceanic late Middle Jurassic to Late Jurassic of the Atlantic Ocean. The oldest zone, the Stephenolithion bigotii Zone (subdivided into a Stephanolithion hexum Subzone and a Cyclagelosphaera margerelii Subzone), is middle Callovian to early Oxfordian. The Vagalapilla stradneri Zone is middle Oxfordian to Kimmeridgian. The Conusphaera mexicana Zone, subdivided into a lower Hexapodorhabdus cuvillieri Subzone and a Polycostella beckmannii Subzone, is the latest Kimmeridgian to Tithonian. Direct correlation of this zonation with the boreal zonation established for Britain and northern France (Barnard and Hay, 1974; Medd, 1982; Hamilton, 1982) is difficult because of poor preservation resulting in low diversity for the cored section at Site 534 and a lack of Tithonian marker species in the boreal realm. Correlations based on dinoflagellates and on nannofossils with stratotype sections (or regions) give somewhat different results. Dinoflagellates give generally younger ages, especially for the Oxfordian to Kimmeridgian part of the recovered section, than do nannofossils.

Sedimentation rates and isotope data of Pyrgo murrhina from DSDP Hole 14-141

Oxygen and carbon stable isotope data of Pyrgo murrhina and flux rates of calcium carbonate in the bio- and magnetostratigraphically dated sediment sequence at DSDP Site 141 were used for a reconstruction of the deep-water circulation in the Northeast Atlantic during Late Miocene and Pliocene times. A distinct change towards reduced advection of deep water recorded near 5.4 Ma is contemporaneous with the cessation of the outflow of the saline Mediterranean water into the Atlantic. During the Pliocene, between 4.5 and 2.75 Ma and between 2.1 and 1.8 Ma, North Atlantic Deep Water (NADW) circulation was sluggish and Site 141 possibly influenced by Antarctic Bottom Water (AABW). Near 2.75 Ma, the advection of well-oxidized NADW was strongly intensified. This change is related to an onset of major Arctic ice growth and/or a major cooling of NADW.

Miocene to Pleistocene sedimentation at ODP Site 133-823

More than 2000 turbidite, debris-flow, and slump deposits recovered at Site 823 record the history of the Queensland Trough since the middle Miocene and provide new insights about turbidites, debris flow, and slump deposits (herein termed gravity deposits). Changes in the composition and nature of gravity deposits through time can be related to tectonic movements, fluctuations in eustatic sea level, and sedimentological factors. The Queensland Trough is a long, relatively narrow, structural depression that formed as a result of Cretaceous to Tertiary rifting of the northeastern Australia continental margin. Thus, tectonics established the geometry of this marginal basin, and its steep slopes set the stage for repeated slope failures. Seismic data indicate that renewed faulting, subsidence, and associated tectonic tilting occurred during the early late Miocene (continuing into the early Pliocene), resulting in unstable slopes that were prone to slope failures and to generation of gravity deposits. Tectonic subsidence, together with a second-order eustatic highstand, resulted in platform drowning during the late Miocene. The composition of turbidites reflects their origin and provides insights about the nature of sedimentation on adjacent shelf areas. During relative highstands and times of platform drowning, planktonic foraminifers were reworked from slopes and/or drowned shelves and were redeposited in turbidites. During relative lowstands, quartz and other terrigenous sediment was shed into the basin. Quartzose turbidites and clay-rich hemipelagic muds also can record increased supply of terrigenous sediment from mainland Australia. Limestone fragments were eroded from carbonate platforms until the drowned platforms were buried under hemipelagic sediments following the late Miocene drowning event. Bioclastic grains and neritic foraminifers were reworked from neritic shelves during relative lowstands. During the late Pliocene (2.6 Ma), the increased abundance of bioclasts and quartz in turbidites signaled the shallowing and rejuvenation of the northeastern Australia continental shelf. However, a one-for-one relationship cannot be recognized between eustatic sea-level fluctuations and any single sedimentologic parameter. Perhaps, tectonism and sedimentological factors along the Queensland Trough played an equally important role in generating gravity deposits. Turbidites and other gravity deposits (such as those at Site 823) do not necessarily represent submarine fan deposits, particularly if they are composed of hemipelagic sediments reworked from drowned platforms and slopes. When shelves are drowned and terrigenous sediment is not directly supplied by nearby rivers/point sources, muddy terrigenous sediments blanket the entire slope and basin, rather than forming localized fans. Slope failures affect the entire slope, rather than localized submarine canyons. Slopes may become destabilized as a result of tectonic activity, inherent sediment weaknesses, and/or during relative sea-level lowstands. For this reason, sediment deposits in this setting reflect tectonic and eustatic events that caused slope instabilities, rather than migration of different submarine fan facies.

Linear sedimentation rates and approximate mass accumulation rates at DSDP Site 61-462

A 560-meter-thick sequence of Cenomanian through Pleistocene sediments cored at DSDP Site 462 in the Nauru Basin overlies a 500-meter-thick complex unit of altered basalt flows, diabase sills, and thin intercalated volcaniclastic sediments. The Upper Cretaceous and Cenozoic sediments contain a high proportion of calcareous fossils, although the site has apparently been below the calcite compensation depth (CCD) from the late Mesozoic to the Pleistocene. This fact and the contemporaneous fluctuations of the calcite and opal accumulation rates suggest an irregular influx of displaced pelagic sediments from the shallow margins of the basin to its center, resulting in unusually high overall sedimentation rates for such a deep (5190 m) site. Shallow-water benthic fossils and planktonic foraminifers both occur as reworked materials, but usually are not found in the same intervals of the sediment section. We interpret this as recording separate erosional interludes in the shallow-water and intermediate-water regimes. Lower and upper Cenozoic hiatuses also are believed to have resulted from mid-water events. High accumulation rates of volcanogenic material during Santonian time suggest a corresponding significant volcanic episode. The coincidence of increased carbonate accumulation rates during the Campanian and displacement of shallow-water fossils during the late Campanian-early Maestrichtian with the volcanic event implies that this early event resulted in formation of the island chains around the Nauru Basin, which then served as platforms for initial carbonate deposition.

Pleistocene sedimentation in the Benguela Current, southeastern Atlantic

We infer variations in paleoproductivity and eolian input at ODP Site 1082 in the Walvis Basin from stable oxygen isotope compositions of the planktonic foraminifera Globorotalia inflata, total organic carbon mass accumulation rates (TOC MAR), and X-ray fluorescence analyses of Fe content. The most pronounced paleoclimatic changes correspond to the time at about 0.9 Ma, when glacial conditions in the northern hemisphere (NH) led to the onset of pronounced 100-kyr glacial-interglacial cycles. We used Fe intensity as a proxy for eolian terrigenous input, and TOC MAR as a paleoproductivity indicator. Paleoproductivity and eolian input show generally higher-amplitude variations of glacial-interglacial cyclicity from 1.5 to 0.58 Ma, indicating pronounced variations in upwellingfavorable winds in this area. At 0.58 Ma, paleoproductivity and eolian input shifted abruptly to lower-amplitude variations with a periodicity of 100 kyr while delta18O values show a trend toward more negative isotope values for the past 0.65 Myr. Especially during glacial periods, oxygen isotope values indicate increasingly warmer sea-surface temperatures toward the end of the Pleistocene. To evaluate the relative influences of NH glaciation and southern hemisphere (SH) insolation as potential forcing mechanisms for variations of eolian input and productivity in the northern Benguela system, we filtered our proxy records at orbital frequencies. The filtered records of Fe intensity and TOC MAR indicate a strong influence of the 100-kyr and 41-kyr frequency bands, supporting our assumption that strong ice buildup in the NH is the dominant trigger for climate changes on the continent and probably in trade-wind intensity. SH insolation and low-latitude precession-related insolation changes were important for paleoproductivity variations in the northern Benguela system, modifying the nutrient supply by southern ocean intermediate waters and the zonal direction of upwelling-inducing trades by the African monsoon system, respectively.