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

Abstract

The late Miocene‐early Pliocene was a time of global cooling and the development of modern meridional thermal gradients. Equatorial Pacific sea surface conditions potentially played an important role in this global climate transition, but their evolution is poorly understood. Here we present the first continuous late Miocene‐early Pliocene (8.0–4.4 Ma) planktic foraminiferal stable isotope records from eastern equatorial Pacific Integrated Ocean Drilling Program Site U1338, with a new astrochronology spanning 8.0–3.5 Ma. Mg/Ca analyses on surface dwelling foraminifera Trilobatus sacculifer from carefully selected samples suggest that mean sea surface temperatures (SSTs) are ~27.8 ± 1.1°C (1σ) between 6.4 and 5.5 Ma. The planktic foraminiferal δ18O record implies a 2°C cooling between 7.2 and 6.1 Ma and an up to 3°C warming between 6.1 and 4.4 Ma, consistent with observed tropical alkenone paleo‐SSTs. Diverging fine‐fraction‐to‐foraminiferal δ13C gradients likely suggest increased upwelling between 7.1–6.0 and 5.8–4.6 Ma, concurrent with the globally recognized late Miocene Biogenic Bloom. This study shows that both warm and asymmetric mean states occurred in the equatorial Pacific during the late Miocene‐early Pliocene. Between 8.0–6.5 and 5.2–4.4 Ma, low east‐west δ18O and SST gradients and generally warm conditions prevailed. However, an asymmetric mean climate state developed between 6.5 and 5.7 Ma, with larger east‐west δ18O and SST gradients and eastern equatorial Pacific cooling. The asymmetric mean state suggests stronger trade winds developed, driven by increased meridional thermal gradients associated with global cooling and declining atmospheric pCO2 concentrations. These oscillations in equatorial Pacific mean state are reinforced by Antarctic cryosphere expansion and related changes in oceanic gateways (e.g., Central American Seaway/Indonesian Throughflow restriction).

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

“…Similar “isotope ordering,” where coccolith carbonate δ 13 C and δ 18 O resembles that of “deep dwelling” nonphotosymbiont‐hosting foraminifera rather than “surface dwelling” photosymbiont‐hosting foraminifera, has been found at other locations within the EEP since the Miocene (Beltran et al, ; Drury et al, ). More interestingly, this has been documented at locations outside the EEP and during older time intervals (Bolton et al, ; Ennyu et al, ; Grant & Dickens, ; Hermoso, ; Mead et al, ).…”

“…In particular, a nominal 1 ‰ overall drop in δ 18 O happened since about 4.3 Ma, and 0.5 to 1.5 ‰ variations in δ 13 C and δ 18 O that increase in amplitude toward present day seemingly happened at 50-100 kyr intervals. Existing stable isotope records of planktic foraminifera from the EEP, including at Site 851, document similar long-term and short-term variations in δ 13 C and δ 18 O (Cannariato & Ravelo, 1997;Drury et al, 2018;Keigwin, 1979). These have been linked to secular changes (long-term) and superimposed orbital pacing in surface water chemistry and temperature.…”

“…We have purposefully steered away from paleoceanographic interpretations of our findings. One can certainly suggest, though, that variations in our bulk carbonate δ 13 C and δ 18 O records at Site 851, and those at other locations in the EEP, relate to variations in surface water temperature, biological productivity and upward advection of deep water (Dickens & Owen, ; Drury et al, ; Reghellin et al, ; Shackleton et al, ). In particular, there were relatively cold waters and greater upwelling at Site 851 during the late Miocene and early Pliocene relative to the middle Pliocene and present day.…”

“…We adopted this age model because the modified one given by Cannariato and Ravelo () does not extend beyond 5 Ma. However, absolute ages for samples using both schemes are offset (between 50 and 200 kyr in the early Pliocene to late Miocene) from current time scales because of misaligned core gaps and overlaps between holes, and imprecise orbital tuning (Lourens et al, ; Lourens et al, ; Drury et al, ).…”

“…A series of studies propose that bulk sediment δ 13 C and δ 18 O values primarily represent the isotopic composition and temperature of the mixed layer, reflecting coccolithophore habitat in the photic zone (Anderson & Steinmetz, 1981, 1983Drury et al, 2018;Reghellin et al, 2015;Shackleton & Hall, 1995;Steinmetz, 1994).…”