The temperature, circulation, and chemical composition of the oceans in the past (the paleocean) differed greatly from those of today. Understanding changes in the oceans throughout Earth history provides context for present-day ocean changes.
Basic Knowledge
Paleoceanography is reconstructed mainly through chemical analyses of foraminiferal and mollusk shells. The δ¹⁸O of foraminiferal shells provides estimates of water temperature and ice volume (seawater volume), while the Mg/Ca ratio provides an estimate of seawater temperature with the influence of ice volume removed. The boron isotope ratio of foraminiferal shells is an indicator of past seawater pH.
Changes in the oceans throughout Earth history:
・Archean to Proterozoic: seawater was in a reducing environment (low in oxygen). At times, the deep ocean was rich in sulfides (euxinic conditions).
・Paleozoic: ocean oxygenation progressed, and diverse benthic organisms flourished.
・Cretaceous: under a warm climate, ocean circulation differed from today, and deep water was warm and low in oxygen; several oceanic anoxic events (OAE) occurred.
・Cenozoic: as cooling progressed, Antarctic bottom water formed, and deep circulation (thermohaline circulation) approached its present pattern.
Changes in the carbon isotope ratio (δ¹³C) are also important indicators, reflecting ocean productivity, the amount of buried organic carbon, and exchange with atmospheric CO₂. Large negative δ¹³C anomalies indicate a large supply of organic carbon and are recorded in events such as the Paleocene–Eocene Thermal Maximum (PETM).
Using a comparison with δ¹⁸O, explain why the Mg/Ca ratio of foraminiferal shells is useful as an indicator of past water temperature.
Answer: δ¹⁸O depends on both seawater temperature and ice volume (the δ¹⁸O of seawater), so ice volume must be corrected for in order to separate water temperature. In contrast, the Mg/Ca ratio is less affected by ice volume because Mg incorporation into foraminiferal shells is a function of water temperature, making it a more direct indicator of temperature change. Combining Mg/Ca with δ¹⁸O makes it possible to separate water temperature and ice volume.
- Foraminiferal δ¹⁸O: combined indicator of water temperature and ice volume
- Foraminiferal Mg/Ca ratio: more direct indicator of seawater temperature
- Boron isotope ratio: indicator of past seawater pH
- Archean: reducing ocean and euxinic conditions
- Cretaceous: warm climate and several oceanic anoxic events (OAE)
- Cenozoic: cooling, formation of Antarctic bottom water, and present-type thermohaline circulation
- Negative δ¹³C anomalies: events involving a large supply of organic carbon
Points to Note
① δ¹⁸O alone cannot distinguish water temperature from ice volume, so it must be combined with complementary indicators such as Mg/Ca. ② Oceanic anoxic events (OAE) differed in scale and duration from present-day human-caused ocean deoxygenation, but they provide useful information for understanding the mechanism. ③ Present-day ocean circulation (thermohaline circulation) has not remained constant throughout Earth history; it differed greatly during the Cretaceous.
Practice
- Why is the foraminiferal Mg/Ca ratio considered a more direct indicator of water temperature than δ¹⁸O?
- What are the episodes of oceanic oxygen depletion that occurred several times during the Cretaceous called?
- What does a large negative anomaly in the carbon isotope ratio (δ¹³C) indicate?