Calcification depth of deep-dwelling planktonic foraminifera from the eastern North Atlantic constrained by stable oxygen isotope ratios of shells from stratified plankton tows

Abstract

<p><strong>Abstract.</strong> Stable oxygen isotopes (<span class="inline-formula"><i>δ</i><sup>18</sup>O</span>) of planktonic foraminifera are one of the most used tools to reconstruct environmental conditions of the water column. Since different species live and calcify at different depths in the water column, the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> of sedimentary foraminifera reflects to a large degree the vertical habitat and interspecies <span class="inline-formula"><i>δ</i><sup>18</sup></span>O differences and can thus potentially provide information on the vertical structure of the water column. However, to fully unlock the potential of foraminifera as recorders of past surface water properties, it is necessary to understand how and under what conditions the environmental signal is incorporated into the calcite shells of individual species. Deep-dwelling species play a particularly important role in this context since their calcification depth reaches below the surface mixed layer. Here we report <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> measurements made on four deep-dwelling <i>Globorotalia</i> species collected with stratified plankton tows in the eastern North Atlantic. Size and crust effects on the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> signal were evaluated showing that a larger size increases the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> of <i>G. inflata </i>and <i>G. hirsuta, </i>and a crust effect is reflected in a higher <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> signal in <i>G. truncatulinoides. </i>The great majority of the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> values can be explained without invoking disequilibrium calcification. When interpreted in this way the data imply depth-integrated calcification with progressive addition of calcite with depth to about 300 m for <i>G. inflata</i> and to about 500 m for <i>G. hirsuta</i>. In <i>G. scitula</i>, despite a strong subsurface maximum in abundance, the vertical <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> profile is flat and appears dominated by a surface layer signal. In <i>G. truncatulinoides</i>, the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> profile follows equilibrium for each depth, implying a constant habitat during growth at each depth layer. The <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> values are more consistent with the predictions of the Shackleton (1974) palaeotemperature equation, except in <i>G. scitula</i> which shows values more consistent with the Kim and O'Neil (1997) prediction. In all cases, we observe a difference between the level where most of the specimens were present and the depth where most of their shell appears to calcify.</p>

Morphotype and Crust Effects on the Geochemistry of Globorotalia inflata

“…Indeed, the largest and clearest difference in δ 18 O due to crusting is observed in core GeoB13862-1 from within the highly dynamic Brazil-Malvinas Confluence Zone (Figure 5). This pattern is consistent with observations from the North Atlantic, where a difference between encrusted and non-encrusted δ 18 O of G. inflata was observed near the Azores Front, but not along the Iberian Margin (Rebotim et al, 2019). It is also consistent with the absence of a crust effect on δ 13 C, even though δ 13 C in planktonic foraminifera is more difficult to interpret (Jonkers et al, 2013;Spero et al, 1997) and we lack information on the shape of the regional vertical profile of δ 13 C of dissolved inorganic carbon.…”

“…Our results from plankton tows further substantiate other observations on the incidence of crusting in the water column (Rebotim et al, 2019); even though encrusted specimens are more common at greater depth, they are not restricted to great depths only (Figure 7). The higher Mg/Ca ratios of specimens collected closer to the sea surface also indicates that the shallow encrusted specimens are unlikely to have been mixed upwards, but rather calcified at higher temperatures at the depths where they were found.…”

Stable Oxygen Isotope Composition Is Biased by Shell Calcification Intensity in Planktonic Foraminifera

“…Our data suggest that the observed effect is mainly an environmental effect, where heavier calcified shells would occupy a deeper habitat of the water column and are thus exposed to lower temperatures of their ambient seawater. The effect size will depend on the species (range of depth habitat; compare Rebotim et al, 2019), the range of shell calcification intensity, and the local oceanography.…”

“…In fact, even through fluctuation of the Azores front, salinities beyond 36.2 are practically never observed in this oceanographic setting (Fründt & Waniek, 2012; Pérez et al., 2003). Temperature changes due to vertical migration, on the other hand, could explain a major part of the measured differences in isotopic compositions for shells with different calcification intensities (Rebotim et al., 2019). The depth habitat of species in the Globigerinoides ruber / elongatus complex is generally shallow, partly due to its dependence on sunlight for photosynthesis, but both living and calcification depths were shown to be reasonably variable within the uppermost 50–100 m of the water column (Lessa et al., 2019; Meilland et al., 2019; Rebotim et al., 2017; Steinhardt et al., 2015).…”

“…The effect size will depend on the species (range of depth habitat; cf. Rebotim et al., 2019), the range of shell calcification intensity, and the local oceanography.…”