Markus Raitzsch

Modern and late Pleistocene B/Ca ratios of the benthic foraminifer Planulina wuellerstorfi determined with laser ablation ICP-MS

Incorporation of boron into foraminiferal shells is thought to be primarily governed by the carbonate chemistry of the ambient seawater, suggesting that it can be reconstructed from B/Ca ratios. To this end, B/Ca ratios of the benthic foraminifer Planulina wuellerstorfi from South Atlantic core top samples have been analyzed using laser ablation−inductively coupled plasma−mass spectroscopy (LA-ICP-MS) to provide additional information on intratest trace element heterogeneity. Results show that boron is heterogeneously distributed within and between shells, with content variations of approximately ±43% displayed within a single shell. B/Ca is higher in the youngest chambers, opposite to the observed between-chamber variability of Mg/Ca. This may be explained by ontogenetic changes of physiological processes that increase the pH of the calcifying fluid and thus the borate concentration while decreasing Mg/Ca to promote calcification. Despite this heterogeneity, mean B/Ca ratios are positively correlated with the deepwater calcite saturation state (Δ[CO32–]), in line with previous studies. We apply this empirical relationship to reconstruct Δ[CO32–] for the late Pleistocene to Holocene using samples from a depth transect in the equatorial Atlantic. Reconstructed Δ[CO32–] values confirm previous studies suggesting that CaCO3-oversaturated North Atlantic Deep Water was reduced during glacial periods, whereas CaCO3-undersaturated Antarctic Bottom Water expanded vertically and propagated northwards. In summary, our data demonstrate that bulk B/Ca in P. wuellerstorfi reliably reflects variations in Δ[CO32–], despite the strong physiological control of boron incorporation.

U/Ca in benthic foraminifers: a proxy for the deep-sea carbonate saturation

The ocean plays a major role in the global carbon cycle, and attempts to reconstruct past changes in the marine carbonate system are increasing. The speciation of dissolved uranium is sensitive to variations in carbonate system parameters, and previous studies have shown that this is recorded in the uranium-to-calcium ratio (U/Ca) of the calcite shells of planktonic foraminifera. Here we test whether U/Ca ratios of deep-sea benthic foraminifera are equally suited as an indicator of the carbonate system. We compare U/Ca in two common benthic foraminifer species (Planulina wuellerstorfi and Cibicidoides mundulus) from South Atlantic core top samples with the calcite saturation state (Δ[CO32−] = [CO32−]in situ − [CO32−]sat) of the ambient seawater and find significant negative correlations for both species. Compared with planktonic foraminifera, the sensitivity of U/Ca in benthic foraminifera to changes in Δ[CO32−] is about 1 order of magnitude higher. Although Δ[CO32−] exerts the dominant control on the average foraminiferal U/Ca, the intertest and intratest variability indicates the presence of additional factors forcing U/Ca.

Hierarchical textures on aragonitic shells of the hyaline radial foraminifer: Hoeglundina elegans

We studied the specific hierarchical architecture of aragonitic shells of a hyaline radial foraminifer, Hoeglundina elegans. The porous shells consist of an ∼50 nm thin top layer and columnar domains ∼1 μm wide and ∼5 μm long in which the c-axis is perpendicular to the surface. The domains are regarded as bundled pillars comprising iso-oriented nanograins ∼5 nm in diameter, and divided by lateral lines of nanoscale voids with an interval of ∼240–500 nm.

Global Synthesis of Sea-Surface Temperature Trends During Marine Isotope Stage 11

To examine the sea-surface temperature (SST) evolution during interglacial Marine Isotope Stage (MIS) 11, we compiled a database of 78 SST records from 57 sites. We aligned these records by oxygen-isotope stratigraphy and subjected them to an Empirical Orthogonal Function (EOF) analysis. The principal SST trend (EOF1) reflects a rapid deglacial warming of the surface ocean in pace with carbon dioxide rise during Termination V, followed by a broad SST optimum centered at ~410 thousand years (ka) before present (BP). The second EOF indicates the existence of a regional SST trend, characterized by a delayed onset of the SST optimum, followed by a prolonged period of warmer temperatures. The proxy-based SST patterns were compared to CCSM3 climate model runs for three time slices representing different orbital configurations during MIS 11. Although the modeled SST anomalies are characterized by generally lower variance, correlation between modeled and reconstructed SST anomalies suggests a detectable signature of astronomical forcing in MIS 11 climate trends.