Paleoceanography

Paleoproxies. Information on Earth's climate in the past is derived from various climate indicators, called Paleoproxies. This includes ice cores, tree rings, corals, sediments and more (for overview, see NOAA). My work has so far focused on proxies in deep-sea sediments. A great deal of what we know about Earth's climatic history is based on fossil shells of organisms called foraminifera.

The geochemical proxy-information preserved in the shells needs to be deciphered and translated into a language that speaks about climate - similar to the deciphering of an ancient language. In many proxies (including foraminifera), the interpretation is complicated by the fact that the primary climate information is recorded by a living organism, which can mask or modify the signal.

We are currently funded by the National Science Foundation to decipher an isotope phenomenon in biogenic CaCO₃ (see Active Research Grants, NSF: OCE05-25647).

For example, at the Last Glacial Maximum (about 18,000 years ago, left), Canada, Scandinavia, and parts of Siberia were covered with thick ice caps and sea-level was approximately 120 m lower than today.

The large swings in temperature and sea-level accompanying the glacial cycles are recorded in the CaCO₃ shells of fossil foraminifera. The oxygen isotope ratio of ¹⁸O to ¹⁶O in their carbonate shells varies as a function of temperature and isotopic composition of the seawater (see Isotope Geochemistry). As a result, the measurement of oxygen isotope ratios in shells of foraminifera from the sediment record has allowed the reconstruction of past climate cycles over millions of years.

The abrupt switches from cold, glacial conditions to warm interglacials (vertical lines) are called terminations. The pacemaker behind these major ice age terminations is still somewhat of a mystery. It has been suggested that the eccentricity variation of Earth's orbit has caused the terminations. However, we consider this to be unlikely. The direct insolation forcing is much too small and eccentricity is also partly out of phase with the onset of the terminations (see figure). For example, Termination III occurs after an eccentricity minimum, Termination VI before; Termination IV occurs half-way between maximum and minimum. Instead, we have proposed that insolation changes occurring in concert in the Northern and Southern Hemisphere have triggered the terminations; see Schulz and Zeebe (2006) under Publications.

PETM. The Paleocene-Eocene Thermal Maximum (PETM) was an extreme warm period in Earth's history that occurred about 55 million years ago. On geologic time scales, the onset of the event was extremely rapid. Earth's surface temperature rose globally by 5-9 degrees Celsius within a few thousand years. Simultaneously, large amounts of carbon were released into the ocean-atmosphere system.

At present, humans are adding large amounts of greenhouse gases to the atmosphere, which leads to warming of the planet's surface. The PETM may turn out to be a valuable lesson from the past on climate change and carbon cycling, which helps us to understand the present and develop better predictions for the future.

The world looked different 55 million years ago. Africa and South America were in closer proximity than today and the basin of the Atlantic Ocean was smaller. An ancient ocean, called the Tethys, separated Europe/Asia from North Africa. Numbers show some sites drilled by the Ocean Drilling Program (ODP) where PETM sections were recovered.

It has been suggested that catastrophic dissociation of oceanic methane hydrate was responsible for the massive carbon release. We are currently investigating scenarios of carbon release during the PETM using sediment and carbon cycle models. Funding for this project is provided by the National Science Foundation (see Active Research Grants, NSF: EAR06-28394).

An adequate data base is the key prerequisite for our modeling efforts. For example, based on deep-sea carbonate records across the PETM (see figure), we were able to show that carbonate dissolution as a response to ocean acidification was not globally homogeneous during the PETM. Rather, dissolution was stronger in the Atlantic compared to the Southern Ocean and Pacific (Zeebe and Zachos, 2007; see Publications).