0
600
Depth (m)
100km
The Baltic Sea
step extractant extracted
1 MgCl
2 (pH8) Exchangeable P
2 CDB (pH~7,5) Fe-bound P
3 Acetate buffer (pH4) Authigenic P
4 HCl (1M) Detrital P
5 Ashing (550°C) + HCl (1M) Organic P
GoF 5 64m
GoF 6 70m
depth (mm) depth (cm)
Fe 2+ Mn 2+
0
10
20
30
40
50
60
0 100 200 300 400 0 100 200 300 400
HPO4 2- SO4 2- NH4 +
depth (mm) depth (cm)
0
10
20
30
40
50
60
0 100 200 300 400 0 2000 4000 6000 8000
CDB-P CDB-Fe CDB-Mn Auth-P Detr-P Org-P
0 100 200 300 0 100 200 300 400 0 100 200 300 0 10 200 10 20 0 10 20 0 2 4 6 8
0 500 1000 1500
-2
-1,5
-1
-0,5
0
-2,5
-2
-1,5
-1
-0,5
0
0 10 20 30 40
Figure 1. Bathymetric map of the Baltic Sea, showing the deep inlet to the GoF. This connectivity allows the halocline and hypoxia of the Baltic proper to penetrate into the GoF.
↓ ↑↓ ↓ ↓
Water column
Org. P → HPO
42- →
← Fe-P
Detr. Ca-P
Oxidized sediment
↓ ↑↓ ↓ ↓
Org. P → HPO
42- ← Fe-P
Detr. Ca-P
Reduced sediment
↓ ↓ ↓
Auth.
Ca-P
↓
Figure 2. Schematic overview of phosphorus forms in oxidized and reduced sediments.
Phosphorus (P) reaches the sediment in two principal forms: as organic material and as detrital apatite. While the detrial apatite is unreactive, a fraction of the organic phosphorus is reminer- alized to HPO4 2- in the sediment. Under oxic conditions, this HPO4 2- binds to Fe-oxides. Under reducing conditions, the Fe-oxides dissolve, releasing HPO4 2-. At depth, when HPO4 2- con-
centrations are high enough, authigenic apatite may precipitate, forming a permanent P sink. A shift towards hypoxia in the water column should increase the regeneration of P by dissolving the shallow-sediment Fe-oxides and potentially increasing the relative remineralization of P
from organic matter. These processes establish a postive feedback in which the P supply for pri- mary productivity is sustained.
Material and methods
GOF 5 and GOF 6 are located at 64m and 70m, respectively, on the northern slope of the Gulf of Finland (Fig. 1). These depths lie within the halocline, and the deeper site GOF 6 experiences more severe and frequent hypoxia. We per- formed sequential phosphorus extractions on cores from the two sites using the scheme of Ruttenberg (1992, see table below) and a complete solid-phase and porewater geochemical characterization.
Introduction
The Gulf of Finland (GoF) is vulnerable to hypoxia due to permanent salinity stratification and high anthropogenic nutrient input. Although the potential ef- fects of bottom water hypoxia on phosphorus regeneration and burial are
known in principle, the GoF presents an unusual combination of geochemical conditions which may influence the importance of these processes relative to the rest of the Baltic. Specifically, GoF sediments are characterized by hypoxic, low salinity porewaters, and a high organic carbon flux. This study aims to es- tablish the effects of these conditions on phosphorus cycling, especially with re- gards to the severity of hypoxia.
Phosphorus cycling in Gulf of Finland sediments
Eefje van Zadelhoff, MSc candidate, University of Utrecht-Geochemistry supervisors: Tom Jilbert and Caroline Slomp
Porewater concentrations (umol/L) Dissolved oxygen
(umol/L): note mm depth scale
Phosphorus species and Fe/Mn oxides (umol/g) C-org (wt%)
Results and Interpretations
The onboard oxygen penetration measurements confirm that GoF 5 is more oxic than GoF 6. Accordingly, the concentration of Fe-P at the core-top is higher at GoF 5 than GoF 6, indicating greater shallow-sediment P retention at GoF 5. Fe-P is also by far the dominant P fraction in the surface sedi-
ments at GoF 5. This retention is confirmed by the HPO4 2- profiles, which show far higher core-top concentrations at GoF 6, thus higher diffusive efllux from the sediments.
The vertical profiles of ‘authigenic’ P suggest that no in-situ precipitation of apatite minerals is taking place; rather, that this fraction is infact detrital in origin. Although the low salinity of the system results in rapid removal of SO4 2- at both sites, there is also no apparent evidence for precipitation of
vivianite (Fe-phosphate) as may occur in association with sulfate depletion elsewhere in the Baltic.
In the absence of authigenic P mineral formation, the dominant burial phase of non-detrital P at both sites is organic-P. The rate of organic P burial is
strongly coupled to the rate of C-org burial, and shows an increasing trend towards the present day. Thus, the expansion of hypoxia in the GoF over the 20th century has increased the rate of P burial as well as P regeneration.
GoF 5
GoF 6
