Water column biogenic silica (Psi) values from sediment traps from R/V Tangaroa cruise 61TG_3052 in the Southern Ocean in 1999 (SOIREE project)

SOIREE Sediment Traps - Water column PSi (biogenic) results<br><br>

<b>METHOD:</b><br>
Biogenic silica content was determined on filtered samples (47 mm , 0.4 um Nuclepore)
after digestion in 0.2 M NaOH, following the methods of Ragueneau and Tréguer (1994).<br>
Estimated accuracy of replicate analyses: &#0177;5-10%<br>
Blank filter: 15 ug<br>
Digest blanks (no filter): indistinguishable from background levels<br>
<br>
<b>NOTES</b><br>
1. The initial and OUTSIDE PATCH water column integrated PSi values from SOIREE were
higher than expected for Southern Ocean HNLC waters. For example, published data from
Bernard Queginuer (France) in the Atlantic sector of the Southern Ocean indicates
maximum BSi concentrations of 1.6 umolSi/l in the vicinity of the Polar Front (53-54&#0176;S),
with maximums in surrounding HNLC waters of 0.25 umolSi/l. SOIREE PSi water column
concentrations reached a maximum of 25 umolSi/l with an average across all profiles of
5.8 umolSi/l and surface values outside the iron-fertilised patch (2-4 umol/l) which were not substantially different from that measured inside the patch during the SOIREE bloom (except on Day T10)
<br><br>
2. There is no obvious indication from either sampling or analytical techniques that the
PSi values are artificially inflated due to sample handling, etc.<br>
<br>
3. Charette & Buesseler (2000, G3) suggest that a previous export event must have occurred
prior to SOIREE due to a substantial 234Th deficit relative to 238U, which might explain
the SOIREE POC results. Since there is no indication from daily SeaWiFS images of a surface
bloom from Nov 1998-Jan 1999 (C. Law pers. comm., 1999), nor any \"memory\" of such an event
in the pCO2 data (Watson et al., 2000, Nature), Charette & Buesseler (2000) suggested that
such a bloom is likely to have been a subsurface feature, or that there was a significant
delay in export<br>
<br>
4. PSi values inside and outside the SOIREE patch were similar despite 6-fold and 3-fold
increases in chlorophyll a concentrations and phytoplankton abundance, respectively, inside
the iron-fertilised patch (Boyd et al., 2000, Nature)<br>
<br>
5. Therefore, we conclude that perhaps there was a significant detrital PSi component in the
water column at the time that the SOIREE site was occupied, representing organic material
derived from a previous bloom/export event.<br>
<br>
6. Given the uncertainties, however, it is wise to treat the interpretation of the above PSi
data with caution.<br>
<br>