Chemistry and accumulation rates of Jurassic to early Cretaceous sediments from the central equatorial Pacific

Sedimentation in the central Pacific during the Jurassic and Early Cretaceous was dominated by abundant biogenic silica. A synthesis of the stratigraphy, lithology, petrology, and geochemistry of the radiolarites in Sites 801 and 800 documents the sedimentation processes and trends in the equatorial central Pacific from the Middle Jurassic through the Early Cretaceous. Paleolatitude and paleodepth reconstructions enable comparisons with previous DSDP sites and identification of the general patterns of sedimentation over a wide region of the Pacific. Clayey radiolarites dominated sedimentation on Pacific oceanic crust within tropical paleolatitudes from at least the latest Bathonian through Tithonian. Radiolarian productivity rose to a peak within 5° of the paleoequator, where accumulation rates of biogenic silica exceeded 1000 g/cm**2/m.y. Wavy-bedded radiolarian cherts developed in the upper Tithonian at Site 801 coinciding with the proximity of this site to the paleoequator. Ribbon-bedding of some radiolarian cherts exposed on Pacific margins may have formed from silicification of radiolarite deposited near the equatorial high-productivity zone where radiolarian/clay ratios were high. Silicification processes in sediments extensively mixed by bioturbation or enriched in clay or carbonate generally resulted in discontinuous bands or nodules of porcellanite or chert, e.g., a "knobby" radiolarite. Ribbon-bedded cherts require primary alternations of radiolarian-rich and clay-rich layers as an initial structural template, coupled with abundant biogenic silica in both layers. During diagenesis, migration of silica from clay-rich layers leaves radiolarian "ghosts" or voids, and the precipitation in adjacent radiolarite layers results in silicification of the inter-radiolarian matrix and infilling of radiolarian tests. Alternations of claystone and clay-rich radiolarian grainstone were deposited during the Callovian at Site 801 and during the Berriasian-Valanginian at Site 800, but did not silicify to form bedded chert. Carbonate was not preserved on the Pacific oceanic floor or spreading ridges during the Jurassic, perhaps due to an elevated level of dissolved carbon dioxide. During the Berriasian through Hauterivian, the carbonate compensation depth (CCD) descended to approximately 3500 m, permitting the accumulation of siliceous limestones at near-ridge sites. Carbonate accumulation rates exceeded 1500 g/cm**2/m.y. at sites above the CCD, yet there is no evidence of an equatorial carbonate bulge during the Early Cretaceous. In the Barremian and Aptian, the CCD rose, coincident with the onset of mid-plate volcanic activity. Abundance of Fe and Mn and the associated formation of authigenic Fe-smectite clays was a function of proximity to the spreading ridges, with secondary enrichments occurring during episodes of spreading-center reorganizations. Callovian radiolarite at Site 801 is anomalously depleted in Mn, which resulted either from inhibited precipitation of Mn-oxides by lower pH of interstitial waters induced by high dissolved oceanic CO2 levels or from diagenetic mobilization of Mn. Influx of terrigenous (eolian) clay apparently changed with paleolatitude and geological age. Cyclic variations in productivity of radiolarians and of nannofossils and in the influx of terrigenous clay are attributed to Milankovitch climatic cycles of precession (20,000 yr) and eccentricity (100,000 yr). Diagenetic redistribution of biogenic silica and carbonate enhanced the expression of this cyclic sedimentation. Jurassic and Lower Cretaceous sediments were deposited under oxygenated bottom-water conditions at all depths, accompanied by bioturbation and pervasive oxidation of organic carbon and metals. Despite the more "equable" climate conditions of the Mesozoic, the super-ocean of the Pacific experienced adequate deep-water circulation to prevent stagnation. Efficient nutrient recycling may have been a factor in the abundance of radiolarians in this ocean basin.

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.

Stable isotopes, CaCO3, opal and terrigeneous of ODP Site 108-663 (Table A1)

High- and low-latitude forcing of terrestrial African paleoclimate variability is demonstrated using 900 ka eolian and biogenic component records from Ocean Drilling Program site 663 in the eastern equatorial Atlantic. Terrigenous (eolian dust) and phytolith (savannah grass cuticle) accumulation rate records vary predominantly at 100 and 41 kyr periodicities and spectral phase estimates implicate high-latitude forcing. The abundance of freshwater diatoms (Melosira) transported from dry African lake beds varies coherently at 23-19 kyr orbital periodicities and at a phasing which implicates low-latitude precessional monsoon forcing. Modeling studies demonstrate that African climate is sensitive to both high- and low-latitude boundary conditions. African monsoon intensity is modulated by direct insolation variations due to orbital precession, whereas remote high-latitude forcing can be related to cool North Atlantic sea surface temperatures (SSTs) which promote African aridity and enhance dust-transporting wind speeds. The site 663 terrigenous and phytolith records covary with North Atlantic SST variability at 41 °N (site 607). We suggest that Pleistocene African climate has responded to both high-latitude North Atlantic SST variability as well as low-latitude precessional monsoon forcing; the high-latitude influence dominates the sedimentary record. Prior to circa 2.4 Ma, terrigenous variations occurred primarily at precessional periodicities (23-19 kyr), indicating that African climate was largely controlled by low-latitude insolation variations prior to the onset of high-amplitude glacial-interglacial climate change.

Accumulation rates, carbon composition and Eocene carbonate compensation depths of ODP Sites 199-1218 and 199-1219

CaCO3, Corg, and biogenic SiO2 were measured in Eocene equatorial Pacific sediments from Sites 1218 and 1219, and bulk oxygen and carbon isotopes were measured on selected intervals from Site 1219. These data delineate a series of CaCO3 events that first appeared at ~48 Ma and continued to the Eocene/Oligocene boundary. Each event lasted 1-2 m.y. and is separated from the next by a low CaCO3 interval of a similar time span. The largest of these carbonate accumulation events (CAE-3) is in Magnetochron 18. It began at ~42.2 Ma, lasted until ~40.3 Ma, and was marked by higher than average productivity. The end of CAE-3 was abrupt and was associated with a large-scale carbon transfer to the oceans prior to warming of high-latitude regions. Changes in carbonate compensation depth associated with CAE excursions were small in the early part of the middle Eocene but increased to as much as 800 m by the late middle Eocene before decreasing into the late Eocene. Oxygen isotope data indicate that the carbonate events are associated with cooling conditions and may mark small glaciations in the Eocene.