59
58
57
56
18 19 20 21 22 23
17 16
0
100
200
300
400
LF1, 67m
BY15, 238m
LL19, 169m
F80, 191m
BY31, 466m Water depth (m)
°E
°N
FÅRÖ DEEP
N. GOTLAND BASIN
LANDSORT DEEP
GOTLAND DEEP
0 20 40 60
0 0.02
0 0.02
Depth (cm)
P
Ca Mn
2 mm
F80, 26cm
F80, 26cm
F80, 26cm (2110)
(32000)
(31703)
59
58
57
56
18 19 20 21 22 23
17
°E 16
°N
18 19 20 21 22 23
17 16
0
100
200
300
400
Water depth (m)
Zone of potential
authigenesis, 100m contour Zone of potential
authigenesis, 175m contour
Oxic interval Anoxic interval
Lateral oxide shuttle
Phosphate mineral authigenesis in anoxic coastal sediments
Tom Jilbert (T.S.Jilbert@uu.nl) and Caroline P. Slomp
Faculty of Geosciences, Utrecht University, The Netherlands
Many modern coastal environments suffer from eutrophication and low-oxygen conditions, as a conse- quence of anthropogenic nutrient loading over the past century. High productivity in these environments is often exacerbated by enhanced regeneration of phosphorus (P) from the underlying sediments, which occurs naturally under low-oxygen conditions (eg., Jilbert et al., 2011). Burial of authigenic P-bearing min- erals constitutes a permanent sink for P in the sediments of anoxic coastal environments. Changes in the rate and distribution of P mineral authigenesis influence the efficiency of P regeneration, and thus may impact upon the recovery of coastal systems from their modern eutrophied state. However, a proper un- derstanding of P mineral authigenesis in anoxic coastal systems is currently lacking.
Introduction
Nature of sedimentary authigenic P minerals
Sequential sediment extractions (Ruttenberg, 1992) yield two authigenic P phases:
CDB-P: mainly iron (Fe)-associated P, eg., P bound to Fe oxyhydroxides, reduced Fe phosphate minerals such as vivianite, Fe3(PO4)2•8(H2O).
Acetate-P: mainly calcium (Ca)-associated P, eg., car- bonate fluorapatite, biogenic apatite, P associated with rhodocrosite (mixed Mn-Ca carbonate)
In the surface sediments of the deep euxinic basins of the Baltic (eg., site F80), CDB-P is enriched relative to shallower sites (Fig. 2). In euxinic conditions, high CDB-P contents must reflect authigenesis of reduced Fe phos- phates such as vivianite. Acetate-P contents are similar at all sites, except for occasional peaks at deep basin sites. This distribution suggests a background flux of apatite from the water column at all sites, and occa- sional P incorporation into rhodocrosite during ventila- tion of the deep basins.
Microanalysis of epoxy-embedded sediments reveals the distribution of the authigenic P phases in the sedi- ments. Discrete 10µm-scale Fe-phosphate particles are present in deep basin sediments (Fig. 3), confirming vivi- anite authigenesis at these locations. Apatite is present within 200µm-scale spherical palynomorphs at all sites and all sediment depths (Fig. 4, left and centre). Layers rich in 200-1000µm-scale globules of P associated with rhodocrosites are found intermittently at deep basin sites (Fig. 4, right).
CDB-P/Al
Acetate-P/Al
0 500 1000 1500 2000 2500
0 2 4 6 8 10
Counts
Energy (keV) Fe P
O
C
P
Fe
300 µm 100 µm
F80, 5cm
F80, 5cm
F80, 5cm
Electron microprobe ED analysis of Fe-P spot (see images to left),
F80, 5cm. Carbon peak is derived from epoxy
embedding medium
0 2 4 6 8 10
Energy (keV) O
P
Ca
Ca
Mn C
500 1000 1500 2000 2500
Counts
0
200 µm LL19, 300cm
F80, 2.5cm
SEM-EDX analysis of apatite concretion within spherical palynomorph (lower left image), LL19, 300cm
0 20 40
Depth (cm) 60
LF1 (67m) LL19 (169m) F80 (191m)
Fig. 3 Microanalysis of Fe-phosphates
Fig. 4 Microanalysis of Mn and Ca-phosphates Fig. 2 Extraction results
Fig. 1 Bathymetric map of the Baltic
Porewater context of vivianite
authigenesis
S (SO4) CH4 HS
Sediment depth (m) 4
2 0
0 10000 0 10000 0 2500
1
3
(µmol/L)
Vivianite authigenesis in marine sediments usually occurs below the sulfate- methane transition zone (eg., Burns 1997) where sulfide is absent and Fe can accumulate in porewaters. However, in the Baltic vivianite precipitates close to the sediment-water interface (Fig. 3) despite the high porewater sulfide and low porewater iron concentra- tions (Fig. 5). This suggests that vivianite precipitation occurs in protected microenviron- ments where Fe-oxyhydrox- ides are reduced in the ab- sence of sulfide, allowing con- version of Fe-oxyhydroxide- bound P to vivianite.
Fe
0 100
Fig. 5 Porewater profiles at F80
Changes in space and time
4 3 2 1 0
0 20
Corg (%)
Sediment depth (m)
0 100
Ptot (µmol/g)
4 3 2 1 0
F80
Medieval LC20
E. Holoc.
Medieval LC20
Phosphate mineral authigenesis at F80 partly accounts for the higher total P concentrations relative to the shallower LL19 site throughout the Holocene (Fig. 6). However, during Baltic-wide anoxic intervals such as the Medieval and Early Holocene periods, authigenic minerals are also observed at LL19, albeit in lower concentrations. Authigenesis of Fe-phosphates and rhodocro- site-P are related to lateral and vertical transport of Fe and Mn oxides, which recharge deep anoxic waters with Fe and Mn.
During generally oxic intervals, Fe and Mn oxides are focused into a few anoxic deep basins, whereas during Baltic-wide anoxia they feed a much larger anoxic zone (Fig. 7).
Fig. 6 Sediment profiles
Fig. 7 Mineral distribution
Oxic interval
Oxic interval
Oxic interval Oxic interval
Oxic interval
LL19
175m depth contour
Micro XRF mapping of P-rich rhodocrosite globules at F80. Numbers in brackets indicate maximum count range
Burns, S. J. (1997) In Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 155.
Jilbert T., Slomp C. P., Gustafsson B. G. and Boer W.
(2011) Biogeosciences 8, 1699-1720.
Ruttenberg K. C. (1992) Limnology and Oceanography 37, 1460-1482.
SUMMARY
Authigenic vivianite and rhodocrosite contribute to P burial in the sediments of anoxic coastal environments such as the Baltic Sea. During most of the Holocene history of the Baltic, their occurrence has been limited to the deep central basins, due to focusing of precursor Fe and Mn oxide phases into these locations. During intervals of widespread anoxia, authigenic P minerals are more extensively distributed, but less concentrated in the sedi- ments. Vivianite forms within microenvironments in the shallow sediments, even when bulk porewater sulfide concentrations predict rapid removal of porewater iron. P-bearing rhodocrosites occur as intermittent layers in the sediments, associated with deep water ventilation events.
REFERENCES
In this study, we investigate phosphate mineral authigenesis in the Baltic Sea (Fig. 1), home to one of the largest euxinic zones in the modern coastal ocean. We use a combination of sediment and porewater geochemical analyses to assess the role of authigenesis in the P cycle of the Baltic, both today and throughout the current brackish phase of Baltic history (~8000 yr BP to present).
Aim
Lateral oxide shuttle
