10:00 AM - 10:15 AM
[BPT07-05] Evaluation of ‘paleo’ red-tide during the mid-Cretaceous Oceanic Anoxic Event (OAE) 2.
Keywords:Cretaceous, Dinoflagellate, red-tide, Oceanic Anoxic Events, acritarch
Values of dinoflagellate production proxy (triaromatic dinosteroid index; TADS) increase at twice during the OAE2 interval, the onset of 1st build-up phase and from the Trough to 2nd build-up phases, in both YG and VB sediments. These results are synchrous with global warming trend estimated by TEX86 and suggest elevated contribution of dinoflagellate for primary production in different two sections located at NW Pacific and Tethys. Huber et al. (1999) reported ‘collapse’ of vertical stratification during the warming phase of the OAE2. These changes related to global warming presumably caused enhanced nutrient supply from intermediate and bottom water masses and global expansion of suitable condition for dinoflagellate. Moreover, marine palynomorphs are mainly composed of very small (< 20µm diameter) spiny acritarchs (Micrhystridium) in the higher TADS samples in both two sections. It was reported that Micrhystridium-accumulated fraction of the Cambrian sediments have high concentration of dinosteane (Talyzina et al., 2000), although the origin of these acritarchs are uncertain. These acritarchs are similar to Gonyaulacoid dinocyst in the morphology and smaller than these cysts, which suggest original species of these acritarch are autotrophic and r strategist such as modern red-tide species. From these results, it is implied that frequent occurrences of red-tides by small dinoflagellate were triggered by global warming and eutrophication, and promoted efficient deposition of organic matter and consequently expansion of anoxic water mass.