Jurassic through Early Cretaceous Sedimentation History of the Central Equatorial Pacific and of Sites 800 and 801

Abstract

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 in...

Modulation of Late Cretaceous and Cenozoic climate by variable drawdown of atmospheric <i>p</i>CO<sub>2</sub> from weathering of basaltic provinces on continents drifting through the equatorial humid belt

“…A nominal mass accumulation rate (MAR) of 1.5 g cm −2 kyr −1 (15 t km −2 yr −1 ) for the Tethyan equatorial belt 5 • S-5 • N was derived from estimates for the Cretaceous equatorial Pacific Ocean (Ogg et al, 1992), which is within the range of values found for the Cenozoic equatorial Pacific (Mitchell and Lyle, 2005;Mitchell et al, 2003).…”

Biotic and environmental dynamics through theLateJurassic–EarlyCretaceous transition: evidence for protracted faunal and ecological turnover

“…Sites with rocks spanning the J/K boundary have been targeted by the International Ocean Discovery Program (IODP) (and earlier incarnations). This program has included sites in (i) the Indian Ocean (Brown, 1992;Gradstein et al, 1992;Kaminski, Gradstein & Geroch, 1992), with evidence for a cooler water regime; (ii) the Pacific Ocean, which is thought to have had a stable circulatory regime (Matsuoka, 1992;Ogg, Karl & Behl, 1992); and (iii) the Atlantic Ocean with a distinct North-South salinity gradient (Deroo, Herbin & Roucaché, 1983;Kotova, 1983;Gradstein et al, 1992). In the Late Jurassic, western Tethys and Atlantic ecosystems were fuelled by a high-nutrient flux, leading to high levels of phytoplankton and radiolarites, possibly driven by shifting circulatory regimes as continental configurations changed (Baumgartner, 1987;Weissert & Mohr, 1996;Danelian & Johnson, 2001).…”

Takuyo-Daisan Guyot: Depositional History of the Carbonate Platform from Downhole Logs at Site 879 (Outer Rim)

“…; 400 k.y.) of Milankovitch climate-oceanographic cooling cycles (e.g., Molinie andOgg, 1992a, 1992b;Ogg et al, 1992). This eccentricity modulation of precession should also cause overall cooling-warming cycles of the deep ocean, thereby creating rises and falls of eustatic sea level (e.g., Revelle, 1990).…”