High‐resolution planktic stable isotope record and spectral analysis for the last 5.35 M.Y.: Ocean Drilling Program Site 625 northeast Gulf of Mexico

Publisher Website
Google Scholar
Abstract
Planktic δ18O, δ13C, and biostratigraphy are presented from a nearly complete Pliocene and Quaternary sequence from Ocean Drilling Program hole 625B, northeast Gulf of Mexico. A high‐resolution chronology is derived for the last 5.35 m.y. by tuning the δ18O record to variations in orbital obliquity. Most ages derived for biostratigraphic datums in hole 625B are similar to the same events based on magnetobiochronologies from outside the basin. The δ18O record indicates that Pliocene and early Pleistocene temperature decrease/ice buildup in the northern hemisphere was nonlinear, occurring via a series of steps and gradual shifts beginning about 3.8 Ma. Late Pliocene (2.30–1.66 Ma) and early Pleistocene (1.66–0.73 Ma) δ18O anomalies with magnitudes of −1.4 to −2.0‰ indicate glacial meltwater discharge from the Mississippi River implying that an ice sheet has been present at mid‐latitudes in North America periodically since 2.3 Ma. Spectral analysis of the δ18O signal indicates that the 100‐kyr period dominated during approximately the last 0.5 m.y. and the 41‐kyr period was dominant during the early Pleistocene and entire Pliocene. A contour plot of the δ18O spectrum indicates that the early to mid‐Pleistocene climate transition commenced about 1.0 Ma with the first occurrence of a strong low‐frequency component to the climate system. This low‐frequency component gradually became the 100‐kyr frequency by about 0.6 Ma. Between 0.7 and 0.6 Ma, the 41‐kyr period decayed and via intermediate frequencies gave way to near‐100‐kyr periodicities. The magnitude of glacial‐interglacial δ13C variation during the middle to late Pleistocene at site 625 is greater than observed in open‐ocean records and may have resulted from increased flux of isotopically light, terrestrial organic carbon into the basin during glacial lowstands of sea level.