Geochemical Archives Encoded in Deep-Sea Sediments Offer Clues for Reconstructing the Ocean's Role in Past Climatic Changes

By Francois, Roger; Bacon, Michael P. | Oceanus, Fall 1997 | Go to article overview
Save to active project

Geochemical Archives Encoded in Deep-Sea Sediments Offer Clues for Reconstructing the Ocean's Role in Past Climatic Changes


Francois, Roger, Bacon, Michael P., Oceanus


Paleoceanographers are trying to understand the causes and consequences of global climate changes that have occurred in the geological past. One impetus for gaining a better understanding of the factors that have affected global climate in the past is the need to improve our predictive capabilities for future climate changes, possibly induced by the rise of anthropogenic carbon dioxide (CO^sub 2^) in the atmosphere.

The relatively recent geological past (the last 1.6 million years), known as the Quaternary period, is characterized by large climatic swings from ice age to warmer interglacial periods similar to the present one. During the ice ages, large ice sheets accumulated on the northern continents, and sea level dropped by as much as 120 meters. Analysis of air bubbles trapped in the Antarctic and Greenland ice caps documents significantly lower atmospheric CO^sub 2^ levels during the cold glacial periods compared to the warm interglacials (Fig. 1). Since carbon dioxide is a well known "greenhouse" gas, whose presence in the atmosphere traps heat near the earth's surface, its lower concentration in the glacial atmosphere could have contributed to the cold climate of the ice ages. Establishing the exact role that atmospheric CO^sub 2^ played in past natural climatic oscillations, however, is not a simple matter. Changes in atmospheric CO^sub 2^ may have been more a response to climate change than a forcing mechanism. On the other hand, while we know that the pace of Quaternary glaciations was primarily driven by variations in Earth's distance from the sun and in the angle of Earth's axis of rotation, the resulting changes in incoming solar radiation to the planet's surface are too small to account for the large climate variability observed. This implies that the effect of these orbital parameters must have been amplified by some internal feedback mechanisms within the earth's environment, and we suspect that atmospheric CO^sub 2^ may be a major factor. In view of its obvious connection to present societal concerns, this particular problem has elicited a lot of attention in the paleoceanographic community.

In the modern ocean, factors affecting atmospheric CO^sub 2^, such as export flux of organic carbon and carbonate to the deep sea, dissolution of calcium carbonate shells in the deep sea, and deep water circulation, can be measured directly. A variety of incubation techniques are used to measure production of organic matter in surface waters, and broad views of surface water production at a given time can now be obtained from satellite imagery (Fig. 2). As several articles in this issue attest, sediment traps are deployed to estimate the export and recycling of organic matter and calcium carbonate from surface waters to the deep sea, and thermohaline circulation is becoming increasingly well constrained, both in terms of flow rates and pathways (see Oceanus Vol. 37, No. 1 and Vol. 39, No. 2). For past oceans, however, these variables cannot be measured directly but must be inferred from proxy analysis (a marker in the sediments from which the variables can be inferred indirectly). A fraction of the biogenic particles produced in surface water survives degradation or dissolution in the deep sea and gradually accumulates on the seafloor. As many oceanic processes leave a chemical imprint in this material, a very complex but rather comprehensive chemical archive, which can be dated and deciphered, is continuously buried in deep-sea sediments.

It is probably safe to say that every element of the periodic table, every isotope, and an assortment of specific organic molecules that survive sediment burial have some potential for providing information on how past oceans operated. It is for us to discover the processes that regulate their distribution in the sediment, how well the chemical signals are preserved during burial, and whether they can then be used to infer past changes in the processes that generated them.

The rest of this article is only available to active members of Questia

Sign up now for a free, 1-day trial and receive full access to:

  • Questia's entire collection
  • Automatic bibliography creation
  • More helpful research tools like notes, citations, and highlights
  • Ad-free environment

Already a member? Log in now.

Notes for this article

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
Loading One moment ...
Project items
Notes
Cite this article

Cited article

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

Cited article

Geochemical Archives Encoded in Deep-Sea Sediments Offer Clues for Reconstructing the Ocean's Role in Past Climatic Changes
Settings

Settings

Typeface
Text size Smaller Larger
Search within

Search within this article

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

While we understand printed pages are helpful to our users, this limitation is necessary to help protect our publishers' copyrighted material and prevent its unlawful distribution. We are sorry for any inconvenience.
Full screen

matching results for page

Cited passage

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

Cited passage

Welcome to the new Questia Reader

The Questia Reader has been updated to provide you with an even better online reading experience.  It is now 100% Responsive, which means you can read our books and articles on any sized device you wish.  All of your favorite tools like notes, highlights, and citations are still here, but the way you select text has been updated to be easier to use, especially on touchscreen devices.  Here's how:

1. Click or tap the first word you want to select.
2. Click or tap the last word you want to select.

OK, got it!

Thanks for trying Questia!

Please continue trying out our research tools, but please note, full functionality is available only to our active members.

Your work will be lost once you leave this Web page.

For full access in an ad-free environment, sign up now for a FREE, 1-day trial.

Already a member? Log in now.

Are you sure you want to delete this highlight?