Paleoclimatology, the study of Earth’s past climates, relies on natural archives to reconstruct environmental conditions that predate human record-keeping. The longest and most continuous of these archives are found in deep-sea ocean sediments, where layers accumulate undisturbed over millions of years. Within these layers are the shells of foraminifera, organisms that act as miniature recorders of the ancient ocean. These tiny fossils preserve chemical signatures of the seawater they lived in, allowing scientists to piece together a history of global climate change. The analysis of these shells provides a perspective on fluctuations in ocean temperature, ice volume, and carbon cycling.
Defining the Microscopic Climate Recorders
Foraminifera, or forams, are protists that construct shells, known as tests, primarily composed of calcium carbonate (CaCO3). This mineral shell material is formed by extracting dissolved ions from the surrounding seawater, locking in the chemical and isotopic composition of the ocean at the time of shell formation. Because their shells are highly sensitive to the chemistry and temperature of the water, forams are considered ideal paleoclimate proxies.
Two main categories of forams are used for climate reconstruction, distinguished by their habitat. Planktonic foraminifera float in the upper water column and record conditions of the surface ocean, such as sea surface temperature. Benthic foraminifera live on or within the seafloor sediment, providing information about the deeper ocean environment, including bottom-water temperatures and nutrient levels. When these organisms die, their shells rain down onto the ocean floor, accumulating layer by layer and creating a chronological record embedded within the sediment.
Extracting the Deep-Sea Archive
Retrieving this climate history involves specialized deep-sea drilling expeditions conducted by international programs. Researchers use coring devices lowered from a ship to penetrate hundreds of meters into the seafloor. The resulting long, cylindrical sediment core brings up layers of mud and fossilized shells that represent continuous time periods.
In the laboratory, the core is split lengthwise, and samples are taken from discrete intervals down the core. Scientists wash the sediment through fine sieves to separate the microscopic foraminifera shells from the bulk mud, typically using a mesh size around 63 micrometers. The shells are then isolated and picked individually under a microscope, often organized by species and size. Since the deeper sections of the core were deposited earlier, the vertical sequence of the sediment directly translates to a timeline of past climate conditions.
Oxygen Isotopes: The Primary Climate Signal
The most important data derived from foraminifera shells come from the ratio of stable oxygen isotopes, Oxygen-18 (O-18) and the lighter Oxygen-16 (O-16). This ratio, expressed as delta-O-18 (δ18O), relates to both global ice volume and water temperature. The CaCO3 shell incorporates these isotopes from the seawater in a ratio that is temperature-dependent, a process known as isotopic fractionation.
Colder water causes the foraminifera to incorporate a slightly higher proportion of the heavier O-18 into their shells. Conversely, warmer water leads to a lower δ18O value in the shell. This relationship means that a change in the shell’s oxygen isotope ratio directly reflects a change in the local water temperature at the time the shell was formed.
The δ18O value in the shells is also strongly influenced by the total volume of ice stored on land. Water molecules containing the lighter isotope, O-16, evaporate more easily from the ocean surface than those with O-18. During cold, glacial periods, this lighter water vapor precipitates as snow and is locked away in ice sheets, leaving the remaining ocean water relatively enriched in the heavier O-18.
Higher δ18O values in foraminifera shells found in deep-sea cores indicate colder ocean temperatures and the presence of large, global ice sheets. The ice volume effect is the dominant control on the deep-sea benthic δ18O record, providing a definitive marker for glacial-interglacial cycles. Paleoclimatologists use this combined signal to map the history of global ice sheet growth and decay.
Secondary Chemical Proxies
To separate the intertwined temperature and ice volume signals recorded by δ18O, scientists rely on secondary geochemical measurements. The Magnesium/Calcium (Mg/Ca) ratio within the foraminifera shells serves as an independent paleothermometer. As the temperature of the surrounding seawater increases, foraminifera incorporate more magnesium ions (Mg2+) into their calcium carbonate structure.
This relationship is exponential, meaning a small temperature change causes a measurable, predictable shift in the Mg/Ca ratio of the shell. By measuring the Mg/Ca of planktonic foraminifera, researchers can reconstruct the absolute sea surface temperature (SST) at the time of calcification. This independent temperature reading allows scientists to calculate the oxygen isotopic composition of the seawater itself, which then reveals the history of global ice volume.
The carbon isotopic ratio (δ13C) reflects changes in the ocean’s carbon cycle and circulation. Surface waters interacting with the atmosphere tend to be enriched in C-13, while deep-water masses accumulate C-12 from the respiration of sinking organic matter.
Benthic foraminifera incorporate the ambient δ13C signal of the deep-water mass they inhabit, making their shells a proxy for tracking the movement and “age” of deep ocean currents. Changes in the δ13C record can be used to map shifts in the strength and pathways of major ocean currents, such as the Atlantic Meridional Overturning Circulation, which are linked to the distribution of heat and nutrients. By combining the primary δ18O signal with these secondary proxies, paleoclimatologists gain a complete picture of Earth’s climate history.