Mg ∕ Ca and <strong><i xmlns="">δ</i></strong><sup>18</sup>O in living planktic foraminifers from the Caribbean, Gulf of Mexico and Florida Straits
<p>Past ocean temperatures and salinities can be approximated from combined stable oxygen isotopes (<span class="inline-formula"><i>δ</i><sup>18</sup>O</span>) and <span class="inline-formula">Mg</span> <span cla...
| Main Authors: | , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-11-01
|
| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/7077/2018/bg-15-7077-2018.pdf |
| Summary: | <p>Past ocean temperatures and salinities can be approximated from
combined stable oxygen isotopes (<span class="inline-formula"><i>δ</i><sup>18</sup>O</span>) and
<span class="inline-formula">Mg</span> <span class="inline-formula">∕</span> <span class="inline-formula">Ca</span> measurements in fossil foraminiferal tests with
varying success. To further refine this approach, we collected living
planktic foraminifers by net sampling and pumping of sea surface water from
the Caribbean Sea, the eastern Gulf of Mexico and the Florida Straits. Analyses
of <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> and <span class="inline-formula">Mg</span> <span class="inline-formula">∕</span> <span class="inline-formula">Ca</span> in eight living planktic
species (<i>Globigerinoides sacculifer</i>, <i>Orbulina universa</i>,
<i>Neogloboquadrina dutertrei</i>, <i>Pulleniatina obliquiloculata</i>,
<i>Globorotalia menardii</i>, <i>Globorotalia ungulata</i>,
<i>Globorotalia truncatulinoides</i> and <i>Globorotalia tumida</i>) were
compared to measured in situ properties of the ambient seawater (temperature,
salinity and <span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>seawater</sub></span>) and fossil tests of
underlying surface sediments. “Vital effects” such as symbiont activity and
test growth cause <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> disequilibria with respect to the
ambient seawater and a large scatter in foraminiferal
<span class="inline-formula">Mg</span> <span class="inline-formula">∕</span> <span class="inline-formula">Ca</span>. Overall, ocean temperature is the most prominent
environmental influence on <span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>calcite</sub></span> and
<span class="inline-formula">Mg</span> <span class="inline-formula">∕</span> <span class="inline-formula">Ca</span>. Enrichment of the heavier <span class="inline-formula"><sup>18</sup>O</span> isotope in
living specimens below the mixed layer and in fossil tests is clearly
related to lowered in situ temperatures and gametogenic calcification.
<span class="inline-formula">Mg</span> <span class="inline-formula">∕</span> <span class="inline-formula">Ca</span>-based temperature estimates of
<i>G. sacculifer</i> indicate seasonal maximum accumulation rates on the
seafloor in early spring (March) at Caribbean stations and later in the year
(May) in the Florida Straits, related to the respective mixed layer
temperatures of <span class="inline-formula">∼26</span> <span class="inline-formula"><sup>∘</sup></span>C. Notably, <i>G. sacculifer</i>
reveals a weak positive linear relationship between foraminiferal derived
<span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>seawater</sub></span> estimates and both measured in situ
<span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>seawater</sub></span> and salinity. Our results affirm the
applicability of existing <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> and <span class="inline-formula">Mg</span> <span class="inline-formula">∕</span> <span class="inline-formula">Ca</span>
calibrations for the reconstruction of past ocean temperatures and
<span class="inline-formula"><i>δ</i><sup>18</sup>O</span><span class="inline-formula"><sub>seawater</sub></span> reflecting salinity due to the
convincing accordance of proxy data in both living and fossil foraminifers,
and in situ environmental parameters. Large vital effects and seasonally
varying proxy signals, however, need to be taken into account.</p> |
|---|---|
| ISSN: | 1726-4170 1726-4189 |