Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

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The equatorial Pacific experienced significant changes in productivity and microfossil assemblage since 16 million years ago (Ma). We compiled a record of those changes from IODP Site U1338 using a reconnaissance of diatom assemblages and high-resolution XRF-scan chemical profiles (1–2 kyr spacing). Productivity and CaCO3 dissolution intervals are defined by sediment component ratios, in particular opal/clay opal/BaSO4 and CaCO3/BaSO4 or CaCO3/clay. There are large abrupt changes in export production in the Miocene, especially compared to the Pleistocene and Pliocene and average levels of export production are higher throughout the Miocene compared to the Pliocene and Pleistocene. Using diatom assemblages and bulk sediment composition, the U1338 sediment record is divided into several distinct deposition regimes: (1) a Middle Miocene Regime > 13.2 Ma marked by low diatom numbers and a few short-lived productivity intervals, (2) a Carbonate Crash Regime distinguished by an older substage (13.2–10.2 Ma) with relatively high abundances of productivity-related diatoms and common productivity-related depositional intervals interspersed with CaCO3 dissolution intervals and a younger Carbonate Crash substage (10.2–8.0 Ma) with weaker brief productivity intervals, high CaCO3 dissolution, and moderately high numbers of upwelling diatom species, (3) the Biogenic Bloom Regime (8.0–4.5 Ma) marked by extended periods of high opal and CaCO3 deposition with a maximum between 7.0 and 6.4 Ma, and (4) a Pliocene–Modern Regime (4.5–0 Ma) with lower productivity and high cyclic CaCO3 dissolution. Changes in production and dissolution result from reorganizations of nutrient supply to the equatorial Pacific, not by higher wind-driven upwelling. We propose that nutrients were more accessible in the Miocene because of a combination of factors including higher organic matter degradation in the upper water column and deep-intermediate nutrient pathways that efficiently recycled nutrients to upwelling zones. Closure of the Central American Seaway was likely a prime cause of the Biogenic Bloom.

Three sites from the equatorial Pacific (ODP 1218, IODP U1333, and U1334) are judged to have a complete stratigraphic sequence across the Eocene–Oligocene boundary. The sections of all three sites from ~40Ma to ~30Ma have been sampled and the samples merged into a single composite section with an average sample spacing <20kyr. A total of 76 radiolarian species, species groups, and specific variant forms provide a detailed radiolarian stratigraphy across this climatically important boundary. Altogether 47 Eocene radiolarian species had their last appearance between ~40Ma and the end of the final step of the Eocene–Oligocene climate transition, many of them in discreet episodes of extinction that grew in magnitude as the transition was approached. First appearance datums appear to cluster at the base of the cool-down from the middle Eocene climate optimum and near the base of the Oligocene, following a maximum in radiolarian last appearances and coincident with relatively abundant diatoms. There appears to be little relationship between paleoproductivity as measured by barite accumulation rates and both first and last appearances of radiolarians. Diatoms are not common in the Eocene and their blossoming in the Oligocene is preceded by the major episodes of radiolarian extinctions. In the Oligocene radiolarian extinctions appear to be associated with low to moderate diatom abundance, whereas species first appearances seem to be associated with moderate to high diatom abundance. Neither paleoproductivity nor a competition for the dissolved silica (a resource necessary for diatom frustule and radiolarian test construction) appears to control the episodic extinction of radiolarians in the Eocene or the more dispersed loss of species in the Oligocene.

There is general agreement that productivity in high latitudes increased in the late Eocene and remained high in the early Oligocene. Evidence for both increased and decreased productivity across the Eocene-Oligocene transition (EOT) in the tropics has been presented, usually based on only one paleoproductivity proxy and often in sites with incomplete recovery of the EOT itself. A complete record of the Eocene-Oligocene transition was obtained at three drill sites in the eastern equatorial Pacific Ocean (ODP Site 1218 and IODP Sites U1333 and U1334). Four paleoproductivity proxies that have been examined at these sites, together with carbon and oxygen isotope measurements on early Oligocene planktonic foraminifera, give evidence of ecologic and oceanographic change across this climatically important boundary. Export productivity dropped sharply in the basal Oligocene (similar to 33.7Ma) and only recovered several hundred thousand years later; however, overall paleoproductivity in the early Oligocene never reached the average levels found in the late Eocene and in more modern times. Changes in the isotopic gradients between deep- and shallow-living planktonic foraminifera suggest a gradual shoaling of the thermocline through the early Oligocene that, on average, affected accumulation rates of barite, benthic foraminifera, and opal, as well as diatom abundance near 33.5Ma. An interval with abundant large diatoms beginning at 33.3Ma suggests an intermediate thermocline depth, which was followed by further shoaling, a dominance of smaller diatoms, and an increase in average primary productivity as estimated from accumulation rates of benthic foraminifera.

This chapter contains sections titled: Introduction Background Methodology and Approach Preliminary Results Volcanogenic Sediments Summary and Conclusions

This chapter contains sections titled: Introduction Spatial and Temporal Framework Percent Opal Data Eocene/Oligocene Transition Late Middle Miocene to Late Miocene Changes in Opal Sedimentation Summary

Census data of a major Cenozoic calcareous nannofossil genus (Discoaster) have been acquired from Site U1338, located near the Equator in the eastern Pacific Ocean and drilled in 2009 during Integrated Ocean Drilling Program (IODP) Expedition 321. The investigated 147.53 m thick upper Miocene sediment sequence is primarily composed of biogenic carbonate and biogenic silica. Diatom biostratigraphic data were used to develop a revised biomagnetostratigraphic age model, resulting in more variable late Miocene sedimentation rates. Carbonate content variations mainly reflect dilution by biogenic silica production, although intense carbonate dissolution affects a few shorter intervals. Abundance variations of discoasters show no distinct correlation with either carbonate or biosilica contents. The two dominant Discoaster taxa are D. brouweri and D. variabilis, except for a 12 m thick interval where D. bellus outnumbers the sum of all other discoasters by a factor of 4.6. Data presented indicate that first D. hamatus and then D. berggrenii both evolved from D. bellus. Three unusual morphotypes, here referred to as Discoaster A, B and C, increase in relative abundance during episodes of enhanced biosilica production in the upper half of the investigated sequence (Messinian). Strikingly similar morphotypes have been observed previously in Messinian age sediments from the Mediterranean, characterized by alternating deposition of biogenic carbonate and biosilica. This suggests a species-specific response among some of the late Miocene discoasters to broader oceanographic and climatic forcing that promoted episodes of enhanced deposition of biogenic silica.

The equatorial Pacific is an important part of the global carbon cycle and has been affected by climate change through the Cenozoic (65 Ma to present). We present a Miocene (12–24 Ma) biogenic sediment record from Deep Sea Drilling Project (DSDP) Site 574 and show that a CaCO3 minimum at 17 Ma was caused by elevated CaCO3 dissolution. When Pacific Plate motion carried Site 574 under the equator at about 16.2 Ma, there is a minor increase in biogenic deposition associated with passing under the equatorial upwelling zone. The burial rates of the primary productivity proxies biogenic silica (bio‐SiO2) and biogenic barium (bio‐Ba) increase, but biogenic CaCO3 decreases. The carbonate minimum is at ∼17 Ma coincident with the beginning of the Miocene climate optimum; the transient lasts from 18 to 15 Ma. Bio‐SiO2 and bio‐Ba are positively correlated and increase as the equator was approached. Corg is poorly preserved, and is strongly affected by changing carbonate burial. Terrestrial 232Th deposition, a proxy for aeolian dust, increases only after the Site 574 equator crossing. Since surface production of bio‐SiO2, bio‐Ba, and CaCO3 correlate in the modern equatorial Pacific, the decreased CaCO3 burial rate during the Site 574 equator crossing is driven by elevated CaCO3 dissolution, representing elevated ocean carbon storage and elevated atmospheric CO2. The length of the 17 Ma CaCO3 dissolution transient requires interaction with a ‘slow’ part of the carbon cycle, perhaps elevated mantle degassing associated with the early stages of Columbia River Basalt emplacement.

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.