Assessing shallow marine biodiversity patterns and climate change using micropaleontological records

Abstract

Shallow marine biodiversity patterns are governed by complicated combinations of environmental factors, yet our understanding on the controlling mechanisms remains limited. In this thesis, microfossils were used as model systems to look for the controlling factors on biodiversity patterns in a submarine cave in Okinawa and Western North Atlantic Ocean’s continental shelf.

For the submarine cave study, the ostracods’ faunal compositions and species diversity were examined from two sediment cores obtained from the Daidokutsu Cave in Okinawa. Together with the results from foraminiferans and bivalves, the study showed gradual reduction of connectivity with the external environment of the cave in the past 7000 years. Also, a cave endemic genus Tabukicypris was discovered from the Daidokutsu Cave, which illustrated a progressive increase in abundance up the sediment cores. Its morphological characteristics were believed to be adaptations to the cave environment, and thus, the rise in abundance further suggested the cave environment had been changing. Even though the sedimentation in the cave is continuous and sheltered from external wave actions, the semi-isolated cave ecosystem is not immune to rapid climate change. It was found that the variability of ostracod species diversity was, in some ways, similar to the intensified East Asian Winter Monsoon variability.

Regarding the large-scale biodiversity pattern in the western North Atlantic, modern ostracod biodiversity was analyzed from 229 sites along the east coast of North America. A negative diversity gradient from the tropics to the Arctic was observed. Our regression models and model-averaging results of the latitudinal diversity patterns showed that bottom water temperature is a significant controlling factor of ostracod diversity, whereas bottom salinity, bottom pH, bottom dissolved oxygen and primary productivity may not be.

Since water temperature is found to be an important controlling factor for modern benthic large-scale biodiversity patterns, the modern pattern was further compared with the latitudinal diversity pattern from past interglacial period in the Pliocene. It was found that the Pliocene latitudinal diversity gradient was different from present. The rise in water temperature, in particular for the mid-latitudes to the North Pole, was also found to be a possible explanation for the differences in latitudinal diversity patterns.

The overall results demonstrate that environmental changes, especially climate change, could change spatio-temporal biodiversity and faunal compositional patterns. Ostracods were proven to be excellent proxies to detect ecosystem responses to past environmental changes and modern environmental conditions, and they can be further used for other ecological and geological studies.