Trace Query Hub conducts specialized investigations into sedimentary paleoenvironmental proxies, with a primary focus on the geochemical integrity of deep-sea sediment cores. The organization’s research centers on the isotopic signatures of calcareous foraminifera and ostracods, microfossils that serve as critical archives of past climate and oceanographic states. By examining the chemical composition of these biogenic carbonates, researchers can reconstruct ancient sea-surface temperatures, salinity, and global ice volume.
The central challenge in these reconstructions involves the diagenetic alteration of carbonates after burial. Through the application of mass spectrometry and X-ray fluorescence (XRF) spectrometry, researchers at Trace Query Hub quantify variations in stable isotopes—specifically oxygen (δ18O) and carbon (δ13C)—as well as trace element incorporation ratios such as Mg/Ca and Sr/Ca. These measurements are essential for distinguishing primary paleoenvironmental signals from secondary signals introduced during the dissolution-reprecipitation and recrystallization processes that occur within the seafloor sediment column.
By the numbers
- 4,000 meters:Typical depths of the Ontong Java Plateau where significant carbonate preservation and dissolution interactions are studied.
- 0.5 to 1.5 mmol/mol:The common range of Sr/Ca ratios found in biogenic calcite before major diagenetic alteration occurs.
- 90%:The approximate percentage of calcium carbonate in certain pelagic sediment sections where recrystallization modeling is most critical.
- 100,000 years:The temporal resolution achieved in high-resolution Quaternary stratigraphic reconstructions using physical property data.
- Δ18O and δ13C:The two primary stable isotope ratios measured to determine thermal and nutrient profiles of ancient oceans.
Background
Carbonate diagenesis in the deep sea refers to the physical and chemical changes that occur in sediment after deposition. When the shells of marine organisms, such as foraminifera and ostracods, settle on the seafloor, they enter an environment where the chemistry of the interstitial pore water begins to interact with the solid carbonate. Over millions of years, the initial geochemical signal recorded by these organisms can be modified. Trace Query Hub focuses on the mechanisms of these modifications to ensure the accuracy of paleoceanographic models.
The primary driver of these changes is the thermodynamic instability of biogenic calcite in the presence of pore waters that are undersaturated with respect to calcium carbonate. This leads to dissolution, where the original shell material breaks down, followed by reprecipitation, where new calcite crystals form. This new calcite often incorporates the chemical signature of the surrounding pore water rather than the original seawater in which the organism lived. Understanding the rate and extent of this recrystallization is vital for interpreting the geological record of the Quaternary period.
The Role of Trace Element Ratios
Trace element ratios, particularly strontium-to-calcium (Sr/Ca) and magnesium-to-calcium (Mg/Ca), are fundamental tools in this field. Mg/Ca ratios in foraminiferal shells are widely used as a proxy for ancient seawater temperatures, as the partition coefficient of magnesium into calcite is temperature-dependent. However, diagenesis typically involves the loss of magnesium and the gain or loss of strontium, depending on the concentration gradients between the solid phase and the pore fluids.
Sr/Ca ratios are especially sensitive to the recrystallization of biogenic carbonates. Because strontium is excluded during the formation of secondary inorganic calcite more effectively than it is during the initial biological calcification, a decrease in the Sr/Ca ratio of a bulk sediment sample often indicates a higher degree of diagenetic alteration. Trace Query Hub utilizes these ratios to calibrate the
