Strontium isotopes in Pleistocene marine molluscs

Strontium isotopes in Pleistocene marine molluscs

B. Ayling

The ocean's strontium isotope composition has been changing through geologic time, with intervals of both decreasing and increasing 87Sr /86Sr ratio indicated by analysis of deep sea sediment cores. Since the mid-Cretaceous (~100Ma), the general trend has been a gradual increase in the 87Sr /86Sr ratio from 0.70740 to its present day value of 0.70917. The three main sources of strontium input into the oceans are hydrothermal exchange at mid oceanic ridges, river runoff from continental weathering, and release of strontium from the skeletons of marine carbonates due to lowering of sea level and subsequent carbonate exposure and recrystallisation. Mid oceanic ridge-derived strontium typically displays a lower 87Sr /86Sr ratios of 0.703, while river runoff is usually higher (around 0.711) due to weathering of basement rocks which are relatively enriched in radiogenic strontium. Carbonate weathering acts as a buffer to the ocean's changing strontium isotope composition, as it negates the increasing 87Sr /86Sr ratio by contributing 87Sr /86Sr ratios from carbonates formed when the ocean's 87Sr /86Sr was lower.

Because of these changes in marine 87Sr /86Sr, strontium isotopes in fossil material have the potential to date and/or correlate marine sequences. In addition, there has been work into the utilisation of strontium isotopes as paleosalinity indicators where d18O or

d13C paleosalinity interpretations are questionable. Strontium isotope ratios appear unaffected by biological processes or evaporation, and are therefore more likely to remain conservative in the seawater-freshwater mixing zone than d13C or d18O.

Part of my work will entail measurement of strontium isotopes from Pliocene and Pleistocene marine molluscs collected from the Wanganui Basin, New Zealand. The Wanganui Basin contains a ca. 2km thick cyclothemic sedimentary sequence which represents a near continuous record of predominantly marine sediment deposition from the late Pliocene (ca.2.5 Ma) to late Pleistocene. Mollusc samples were collected from 5 units which are chronologically dispersed throughout the section. These are the:

Landguard Formation (~0.25 Ma)
Tainui Shellbed (~0.5 Ma)
Kupe Formation (~0.68 Ma)
Omapu Shellbed (~0.85 Ma)
Hautawa Shellbed. (~2.5 Ma)

The ages of these deposits are already fairly well constrained by the combination of cyclostratigraphy, tephrochronology, biostratigraphy and paleomagnetic studies. It will therefore be interesting to analyse strontium isotopes from these deposits to provide additional age constraints/dating comparisons, and potentially add more data to the Pleistocene section of the ocean's strontium isotope evolution curve which is somewhat lacking in detail.

Macrofossils from these units generally show excellent preservation for the 4 youngest samples, particularly the Tainui Shellbed (see Figure 3).

 

Figure 3: Tainui shellbed fossil assemblage:

A = Ostrea chilensis, B = Talochlamys gemmulata,

C = Purpocardia purpurata, D = Tawera spissa, E = Amalda mucronata,

F = Austrofusus glans, G = Notocallista multistriata.

 XRD was performed on selected species from the Tainui shellbed, and it was found that the molluscs are 99% or greater calcite or aragonite, indicating excellent preservation. In addition, molluscs were examined under a SEM to investigate the  appearance of primary aragonite or calcite crystals. It appears that the exterior portions of the molluscs are the most likely to experience diagenetic recrystallisation whereas nacreous surfaces are commonly pristine (see Figure 4).

 Figure 4: SEM images of selected fossils from the Tainui Shellbed. A = exterior of Purpocardia purpurata, B = pristine primary aragonite in Purpocardia purpurata, C = pristine primary calcite in Talochlamys gemmulata, D = diagenetic randomly oriented calcite crystals in Ostrea chilensis.
 Shell sampling for strontium isotope analysis was undertaken using a handheld dental drill. Several sub-samples were collected from each mollusc, both to examine intra-mollusc variation in 87Sr /86Sr (there should be no detectable variation in this ratio over the lifetime of a mollusc), and to attempt to identify areas of a mollusc which preferentially experience dissolution (for example anterior Vs posterior, nacreous interior Vs exterior). The samples were dissolved in weak acetic acid (1N), centrifuged to remove organics and clay material, and the resulting leachate was passed through a strontium specific cation exchange resin. Currently they are being analysed by TIMS (thermal ionisation mass spectrometry) for their strontium isotope composition.