Middle Miocene ocean‐climate transition: High‐resolution oxygen and carbon isotopic records from Deep Sea Drilling Project Site 588A, southwest Pacific

Abstract

High‐resolution stable isotopic records are presented for the epi‐benthic foraminifer Cibicidoides , the inferred shallow‐dwelling planktonic Globigerinoides quadrilobatus , and the inferred deep‐dwelling planktonic Globoquadrina dehiscens from the middle Miocene (∼16–12 Ma) of Deep Sea Drilling Project site 588A, Lord Howe Rise, southwest Pacific. High‐resolution, multiple species oxygen and carbon isotopic data define the timing and character of the well‐known middle Miocene climatic‐oceanographic transition with a resolution comparable to Quaternary records. The benthic foraminiferal δ 18 O record is marked by several large fluctuations from ∼16 to 14.8 Ma, followed by a series of rapid (<50 kyr) δ 18 O increases that suggest a new state of the ocean‐climate system after 14.8 Ma. The total middle Miocene benthic oxygen isotopic increase of 1.2‰ is largely incorporated in two steps, an increase of 0.8‰ from 14.5 to 14.0 Ma and a second increase of 0.7‰ from 13.45 to 12.45 Ma. Each step is comprised of a series of marked δ 18 O increases, indicative of rapid East Antarctic ice sheet growth and contemporaneous deepwater cooling. A strong covariance of 0.7‰ between the benthic and deep‐dwelling planktonic species from 14.5 to 14.0 Ma (including a rapid increase from 14.1 to 14.05 Ma) suggests a 0.7‰ increase in the δ 18 O composition of seawater (δ 18 O sw ) because of East Antarctic ice sheet growth. Comparison of the δ 18 O record of Gs. quadrilobatus suggests that surface waters warmed at this site by ∼3°C from 14.1 to 13.6 Ma. Carbon isotopic time series for each species generally covary throughout the early to middle Miocene interval (∼16–12 Ma), confirming that δ 13 C variations in this interval largely represent reservoir changes. High‐resolution δ 13 C data allow improved resolution of the latter five of six δ 13 C maxima within the well‐known early to middle Miocene carbon isotopic excursion (the Monterey Carbon Isotopic Excursion from 17.0 to 13.5 Ma). This is useful for global correlation. The last of these maxima ends with a 1‰ decrease centered from 13.9 to 13.7 Ma, ∼300 kyr after the δ 18 O increase considered to reflect East Antarctic ice growth. Covariance between benthic δ 18 O and δ 13 C from ∼16 to 13.8 Ma suggests a sensitive relation between global carbon cycling and the ocean‐climate system prior to 13.8 Ma. Episodic increases in organic carbon burial may have contributed to deep‐sea benthic δ 13 C maxima and synchronous global cooling. The positive relationship ended at ∼13.8 Ma, indicative of changing relations between global carbon cycling and the ocean‐climate system brought on by the increased stability of the East Antarctic ice sheet after a major growth phase from 14.5 to 14.0 Ma.