TOKYO, 6.6 – 3.7K YEARS AGO: COOL & DRY SUMMERS, WARM WINTERS - MARINE BIVALVE SHELLS RECONSTRUCT SEASONAL CLIMATE AND ENVIRONMENTAL CONDITIONS

What was the seasonal environmental and climate variability like during the mid/late-Holocene in subtropical settings? Seasonal climate variability during the past 15.000 years, especially during times of profound climatic change, is the focus of many modern climate change studies. In general, data on seasonal climate variability of the recent past can improve atmosphere-ocean general circulation models (GCMs). More precise GCMs are essential to assess the impact of humans on global and regional climate and to predict the future evolution of the atmospheric and the oceanic circulation and thermodynamics. Data on seasonal environmental and climate fluctuations can also clarify possible links between cultural innovation and climate change.

The mid/late Holocene transition is generally regarded as a period of substantial global climate change during the termination of the mid-Holocene climatic optimum. During the “climatic optimum” the climate was perhaps more seasonal than today, i.e., summers in the northern hemisphere were generally warmer, and winters probably cooler. However, little is known, for instance, about the seasonal climate variation in subtropical regions. We also do not know much about the severity and duration of the monsoon and typhoons seasons, or how long a specific temperature regime lasted. Those questions can be answered by analyzing bivalve mollusk shells.

Bivalve mollusk shells provide ultra-high-resolution climate proxies in their shells. Daily growth patterns and the oxygen and carbon isotope composition of modern juvenile, shallow-marine bivalve mollusk shells of Phacosoma japonicum (Reeve) in Japan were analyzed and cross-calibrated with environmental parameters. Fossil shell radicarbon ages range from 6.60 to 3.74kyrs. Preliminary results indicate that the winters at Tokyo Bay during the mid/late Holocene were warmer, whereas the summers were colder and less wet than today; monsoons and typhoons were much less severe or even missing during certain years. High nutrient levels supported fast shell growth rates during the growing season. Nutrient deprivation during the second half of the year limited shell growth.

Our study demonstrates the usefulness of accretionary skeletons for high-resolution climate and environmental reconstructions.