Research topics

Last update: Aug. 29, 2003
Introduction

During the last 200 million years, Earth’s climate has been cooling on the whole. Based on the results from marine sediment cores collected mainly by the Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP), we have realized that the climatic changes during the last 200 million years is a convolution of many climatic variables caused by various mechanisms and periodicity. They include those that originated from geological events like a bolide impact at K-T Boundary, monsoon establishment by the uplift of the Himalayan-Tibetan region, and Antarctic Circum Polar Currents, and periodic climatic change like glacial-interglacial cycles with 20-100 kyr periodicity, and quasi-periodic climatic changes like Dansgaad/Oeschgar Events and Heinrich Events which occurred in the timescales of 10-1000 years. They can be grouped into either internal or external causes; the former includes distribution and rate constants of elemental cycles in the crust and mantle, whereas the latter includes bolide impact and Earth’s orbital forcing. The ultimate goals of our group’s research are to precisely describe these climatic changes during the last 200 million years and to understand the mechanisms driving these changes.


Research Topics
1. Investigations of sedimentary rocks and sediment cores recovered by ocean drilling
1.1. Mechanism for Cretaceous black shale formation
1.2. Biogeochemical processes in the redox boundary
1.3 Climate sensitive oceans in Quaternary “Icehouse Earth”
1.4. Sedimentary processes in high sedimentation area of the continental margin
2. Investigation of paleoenviroments using numerical modeling

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1. Investigations of sedimentary rocks and sediment cores recovered by ocean drilling

How was the present global environment formed? What will happen to the future global environment? To answer these crucial questions, we must understand the system driving the Earth’s climate. To know how Earth’s system has been and will be operated, we must reconstruct the global change in the past on various timescales to identify where we are, and estimate the processes that controlled change. We will achieve this goal by participating and contributing to the science plan of the Integrated Ocean Drilling Program (IODP). [ IODP webpage ] We will investigate the changes in the global system over the last 200 million years by analyzing physical, chemical, and biological properties recorded in the drilling core samples taken by a new riser ship “Chikyu”, a non-riser ship, and a mission specific platform, all of which will be operated by IODP.
1.1. Mechanism for Cretaceous black shale formation

During the Cretaceous period from 145 to 65 million years ago, Earth’s climate was extremely warm without ice caps at both poles. Since the concentration of atmospheric carbon dioxide in the Cretaceous was estimated to have been much higher than the present, we term this period as “Greenhouse Earth”. During this period several organic-rich “black shales” were deposited around the globe. The black shales are now source rocks of petroleum on which modern civilization heavily relies. The geological events were believed to be caused by reducing degradation rate of sedimentary organic matter, which has been referred to as Oceanic Anoxic Events (OAEs). However, detailed processes of the OAEs remain poorly constrained. We are investigating the black shales collected from outcrops in central Italy and southern France. We are also studying the relationship between black shale formation and other Cretaceous events like the formation of large igneous provinces (LIPs) or long stable geomagnetic polarity.

Bonarelli Blackshale

Figure 1　Image of Libello-Bonarelli black shale in Itary
1.2. Biogeochemical processes in the redox boundary

To estimate the Cretaceous OAE environments, we are studying modern anoxic environments similar such as Lake Kaiike in Kamikoshiki Island, Kagoshima Prefecture. In the Lake Kaiike, a sharp redox boundary exists permanently at 5 m depth and a large amount of hydrogen sulfide evolves within the anoxic deepwater of the lake. An extremely dense population of photosynthetic bacteria, such as purple sulfur bacteria or green sulfur bacteria, and chemosynthetic bacteria are present at the redox boundary. At the bottom of the lake, green sulfur bacteria and cyanobacteria form bacterial mats. We are studying the chemical species of dissolved and particulate phases, genetic analysis of symbionts of benthic foraminifera, biomarkers and their stable carbon and hydrogen isotopic compositions, stable iron isotopic compositions, microscopic observation of sedimentary fabric in order to understand biogeochemical processes in the lake.

Kaiike
Figure 2, Schematic illustration of Lake Kaiike (on Is. Kamikosiki, Kagoshima)
At the redox boundary hotosynthetic bacteria such as purple and green sulfur bacteria, and chemoautotrophic bacteria form thick bacterial plate at redox boundary in lake water. At the bottom of lake, surface sediments are covered by bacterial mats composed of green sulfur bacteria and cycnobacteria.
1.3 Climate sensitive oceans in Quaternary “Icehouse Earth”

During the Quaternary, the last 2 million years, the concentration of atmospheric carbon dioxide has been the lowest in the Earth’s history. With a bipolar glaciation, we call the Quaternary climate as “Icehouse Earth”. Recent investigations indicated that the glacial/interglacial cycles driven by Milankovitch orbital cycles are overlapped by millennium-to-centennial scale climate changes like Dansgaard/Oeschger Events and Heinrich Events. We are investigating sediment cores recovered from high-latitudinal regions including the Okhotsk Sea and Ross Sea, Antarctica, areas potentially sensitive to climatic change in order to understand the role of high latitude oceans in future global changes.
　
icehouse
Figure 3: Example of environmental change in Icehouse Earth system.
Sea-ice expansion in the Sea of Okhotsk, representing by increase of ice-rafted debris (IRD), has occurred at the timing of millennium scale colder periods (stadials) obtained by Greenland ice cores.
1.4. Sedimentary processes in high sedimentation area of the continental margin

On the continental margin, sediments accumulate at rates of several meters or more in every thousand years. It is a site for deposition and burial of terrestrial as well as marine organic matter, and it can act as a sink for carbon from the ocean/atmosphere system. Organic matter buried in that region degrades physicochemically and microbiologically, to ultimately form energy resources such as petroleum, natural gas, or gas hydrates. To understand the processes involving the organic matter in these regions, we conducted core drilling at Sagara Oil Field, Shizuoka Prefecture and also plan to participate with IODP in the Nankai Trough. Furthermore, to understand the sedimentary processes and to quantitatively estimate the carbon cycle, we have performed observations and in situ experiments on the seafloor in Sagami Bay (1450 m depth) with colleagues in other institutions. Facies with high sedimentation rates in the continental margin also provides a high-resolution record of the past environmental changes. We are investigating high sedimentation-rate sediments from a drilling core from Choshi, Chiba Prefecture to reconstruct mid-Quaternary climatic changes.
sediment-water_interface 
Figure 4: A model for modern sedimentary processes at sediment/water interface in Sagami Bay.

In Sagami Bay, accumulation rate of the sediments is very high with horizontally advected fine detritus as well as seasonally, vertically transported organic matter. The organic matter derived from surface water is preferencially assimilated, decmposed, and regenerated by benthic organisms.


2. Investigation of paleoenviroments using numerical modeling

In preparation