Coral skeleton P/Ca proxy for seawater phosphate: Multi-colony calibration with a contemporaneous seawater phosphate record

• Published in: Geochimica et cosmochimica acta Vol. 74; no. 4; pp. 1282 - 1293
• Main Authors: LaVigne, Michèle, Matthews, Kathryn A., Grottoli, Andréa G., Cobb, Kim M., Anagnostou, Eleni, Cabioch, Guy, Sherrell, Robert M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15-02-2010; Elsevier
• Subjects: Calibration; Corals; Earth Sciences; Geophysics; Marine; Mathematical analysis; Montastrea; Nutrients; Pavona gigantea; Phosphates; Physics; Porites lobata; Porites lutea; Proxy client servers; Sciences of the Universe; Sea water; Surface water; Panama
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2009.11.002

A geochemical proxy for surface ocean nutrient concentrations recorded in coral skeleton could provide new insight into the connections between sub-seasonal to centennial scale nutrient dynamics, ocean physics, and primary production in the past. Previous work showed that coralline P/Ca, a novel seawater phosphate proxy, varies synchronously with annual upwelling-driven cycles in surface water phosphate concentration. However, paired contemporaneous seawater phosphate time-series data, needed for rigorous calibration of the new proxy, were lacking. Here we present further development of the P/Ca proxy in
Porites lutea and
Montastrea sp. corals, showing that skeletal P/Ca in colonies from geographically distinct oceanic nutrient regimes is a linear function of seawater phosphate (PO
4
SW
) concentration. Further, high-resolution P/Ca records in multiple colonies of
Pavona gigantea and
Porites lobata corals grown at the same upwelling location in the Gulf of Panamá were strongly correlated to a contemporaneous time-series record of surface water PO
4
SW
at this site (
r
2
=
0.7–0.9). This study supports application of the following multi-colony calibration equations to down-core records from comparable upwelling sites, resulting in
±0.2 and
±0.1
μmol/kg uncertainties in PO
4
SW
reconstructions from
P. lobata and
P. gigantea, respectively.
P
/
Ca
Porites
lobata
(
μ
mol
/
mol
)
=
(
21.1
±
2.4
)
PO
4
SW
(
μ
mol
/
kg
)
+
(
14.3
±
3.8
)
P
/
Ca
Pavona
gigantea
(
μ
mol
/
mol
)
=
(
29.2
±
1.4
)
PO
4
SW
(
μ
mol
/
kg
)
+
(
33.4
±
2.7
)
Inter-colony agreement in P/Ca response to PO
4
SW
was good (±5–12% about mean calibration slope), suggesting that species-specific calibration slopes can be applied to new coral P/Ca records to reconstruct past changes in surface ocean phosphate. However, offsets in the
y-intercepts of calibration regressions among co-located individuals and taxa suggest that biologically-regulated “vital effects” and/or skeletal extension rate may also affect skeletal P incorporation. Quantification of the effect of skeletal extension rate on P/Ca could lead to corrected calibration equations and improved inter-colony P/Ca agreement. Nevertheless, the efficacy of the P/Ca proxy is thus supported by both broad scale correlation to mean surface water phosphate and regional calibration against documented local seawater phosphate variations.