JURASSIC-CRETACEOUS d13C COMPILATION: THE SEARCH FOR LINKS AMONG THE CARBON CYCLE, PHYTOPLANKTON EVOLUTION, SEA-LEVEL CHANGE, AND GEOLOGIC PROXIES

We use new and published isotope data to construct a bulk sediment d¹³C curve for the Jurassic and Cretaceous. This compilation shows widespread d¹³C increases in the Pliensbachian-early Toarcian (an OAE), ?Bajocian, Callovian-Oxfordian, mid-Valanginian, early Aptian (OAE1a), late Aptian, Albian (smaller d¹³C increases of OAE 1b, 1c, & 1d), and Cenomanian/Turonian (OAE2). Rapid, transient d¹³C decreases occurred in the Toarcian, Oxfordian, and Tithonian.

Events in published faunal and geologic records often correspond to these d¹³C changes, such as: 1) nannoconid crises in the Valanginian and earliest Aptian; 2) nannofossil extinctions in the Toarcian, Oxfordian, OAE1a, and OAE2; 3) decreases in diatom and dinoflagellate diversity beginning ~OAE2; 4) Sr-isotopic decreases (=probable increases in hydrothermal activity) began immediately prior to the Toarcian, Oxfordian, and OAE2 events; in contrast, a major Sr-decrease post-dates OAE1.

The isotope, faunal, and proxy records compiled here demonstrate that complex interactions among multiple paleoenvironmental parameters affected the evolutionary trends of Mesozoic phytoplankton, ultimately favoring the three orders of eucaryotes that dominate today’s oceans (diatoms, dinoflagellates, coccolithophores). Our compilation highlights a first-order correlation between long-term (10⁷ -10⁸ my) sea-level rise and increased diversity of the eucaryotes at the genus and family level; in contrast, short-term (10⁶ my) sea-level change shows little correlation with evolutionary trends. This indicates that major phytoplankton radiations were related to expanded ecological niches and ocean chemistry changes that accompanied long-term continental shelf flooding and increased sea-floor spreading rates. Superimposed on these long-term trends are times of rapid faunal turnover at the species level that often correspond to rapid changes in isotope and proxy records.