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

• Main Authors: A. Rebotim, A. H. L. Voelker, L. Jonkers, J. J. Waniek, M. Schulz, M. Kucera
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-07-01
• Series: Journal of Micropalaeontology
• Online Access: https://www.j-micropalaeontol.net/38/113/2019/jm-38-113-2019.pdf
• ISSN: 0262-821X; 2041-4978

<p>Stable oxygen isotopes (<span class="inline-formula"><i>δ</i>¹⁸O</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>¹⁸O</span> of sedimentary foraminifera reflects to a large degree the vertical habitat and interspecies <span class="inline-formula"><i>δ</i>¹⁸</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>¹⁸O</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>¹⁸O</span> signal were evaluated showing that a larger size increases the <span class="inline-formula"><i>δ</i>¹⁸O</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>¹⁸O</span> signal in <i>G. truncatulinoides. </i>The great majority of the <span class="inline-formula"><i>δ</i>¹⁸O</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&thinsp;m for <i>G. inflata</i> and to about 500&thinsp;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>¹⁸O</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>¹⁸O</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>¹⁸O</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>