Biogenic sediment regimes in the Neogene equatorial Pacific, IODP Site U1338: Burial, production, and diatom community

Abstract

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 23 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 32 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 35 (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 3 upwelling zones. Closure of the Central American Seaway was likely a prime cause of the Biogenic Bloom.

Understanding Bulk Sediment Stable Isotope Records in the Eastern Equatorial Pacific, From Seven Million Years Ago to Present Day

“…Certainly, numerous high values exist during Int1 (0–0.56 Ma) and numerous high and low values exist during Int 4 (6.40–6.80 Ma). Most low CaCO 3 content values relate to high biosilica (mainly diatom) abundance (Backman et al, ; Farrell et al, ; Lyle & Baldauf, ).…”

Deciphering the State of the Late Miocene to Early Pliocene Equatorial Pacific

“…Prior to and during most of the LMCIS (~8.0–7.1 Ma), the Site U1338 δ 13 C gradients remain comparatively constant: the planktic‐benthic δ 13 C gradient is ~2.1‰, the fine‐benthic δ 13 C gradient is ~0.8‰, and the planktic‐fine δ 13 C gradients is ~1.25‰ (dashed lines, Figure c). The late Miocene Biogenic Bloom (LMBB), also observed in all oceanic basins, is partially coeval with the LMCIS, suggesting that increased global productivity may have driven the LMCIS (Diester‐Haass et al, , ; Expedition 320/321 Scientists, ; Grant & Dickens, ; Lyle et al, ; Lyle & Baldauf, ; Pälike et al, ). At Site U1338, sedimentation rates are generally higher between ~8.0–4.6 Ma (Figure f), supporting the proposed 8.0 to 4.5 Ma timing of the LMBB at Site U1338 (Lyle & Baldauf, ; Reghellin et al, ).…”

“…However, these observations contradict evidence for either oscillating dominant El Niño‐/La Niña‐like conditions during the late Miocene (Nathan & Leckie, ) or stable E‐W equatorial SST gradients for the last 12 Myr (Zhang et al, ). Further complicating understanding of the equatorial Pacific Ocean, is the globally recognized, but not globally synchronous Late Miocene Biogenic Bloom (LMBB), which occurred in both upwelling zones and oligotrophic regions between ~8.0 and 4.5 Ma, and may reflect increased surface ocean nutrient delivery via either increased upwelling of cool nutrient‐rich waters or increased continental runoff (Diester‐Haass et al, , , ; Grant & Dickens, ; Hermoyian & Owen, ; Liao & Lyle, ; Lyle & Baldauf, ; Herbert et al, ). However, a forcing mechanism of increased upwelling (Grant & Dickens, ) is difficult to reconcile with observations of eastern equatorial Pacific surface warmth and a deep E‐W thermocline across the equatorial Pacific Ocean (LaRiviere et al, ).…”

Plio‐Pleistocene glacial‐interglacial productivity changes in the eastern equatorial Pacific upwelling system

“…Different driving mechanisms have been proposed to explain the secular shift toward a more productive regime in low latitudes during Pliocene iNHG. The (sub)tropical productivity increase is suggested to result (i) from a shoaling of the tropical thermocline/upwelling intensification, predominantly depending on the trade wind strength and thus the equator‐to‐pole temperature gradient [ Pisias and Mix , , and references therein; Cleaveland and Herbert , ; Etourneau et al ., ], and/or (ii) from a reorganization of the nutrient content in high southern latitude surface waters [ Lawrence et al ., , ; Bolton et al ., ; Lyle and Baldauf , ] that feed low‐latitude upwelling regions [ Tsuchiya et al ., ; Toggweiler et al ., ]. Over the early and middle Miocene, a major source for the waters upwelled in the EEP was the deep water formed in the higher‐latitude Southern Ocean, as evidenced by small benthic carbon isotope gradients [ Poore et al ., ; Cramer et al ., ] and neodymium isotope records [ Holbourn et al ., ] across the Pacific Ocean basin.…”

“…At the global scale proxy records for export productivity demonstrate high‐latitude productivity shifts [e.g., Hillenbrand and Fütterer , ; Bolton et al ., ] that are broadly coincident with but opposite in sign to (sub)tropical productivity patterns in upwelling regions [e.g., Lawrence et al ., , ; Bolton et al ., ]. The mechanisms driving these low‐latitude productivity fluctuations are a subject of ongoing debate with hypotheses including changes in trade wind strength/upwelling intensity (“upwelling hypothesis”) [see Cleaveland and Herbert , ; Etourneau et al ., ] and/or changes in the nutrient content of the upwelled water masses (“nutrient delivery hypothesis”) [see Lawrence et al ., , ; Bolton et al ., ; Lyle and Baldauf , ].…”