Phosphorus forms and reactive iron in lateglacial, postglacial and brackish-water sediments of the Archipelago Sea, northern Baltic Sea

Marine Geology 252 (2008) 1–12

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Phosphorus forms and reactive iron in lateglacial, postglacial and brackish-water sediments of the Archipelago Sea, northern Baltic Sea Joonas J. Virtasalo a,⁎, Aarno T. Kotilainen b a b

Department of Geology, FI-20014 University of Turku, Finland Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland

A R T I C L E

I N F O

Article history: Received 8 February 2007 Received in revised form 13 March 2008 Accepted 15 March 2008 Keywords: phosphorus reactive iron lacustrine brackish-water glacial sediments anoxic sediments Archipelago Sea Baltic Sea

A B S T R A C T Glacial transport is an important agent of P and reactive Fe delivery to the ocean margins. However, whereas P may travel through salinity gradients, reactive Fe is substantially trapped in estuaries. Studies of glaciolacustrine to brackish-water depositional successions elucidate processes associated with the glacial P and reactive Fe flux into the oceans. The sedimentary P species, ‘reactive’ Fe-oxide-associated Fe (and Al and Mn), C, N and S geochemistry for sediment cores from three sites in the Archipelago Sea (northern Baltic Sea) were determined in order to understand the distribution and coupling of these geochemical components in the glaciolacustrine to post-glacial lacustrine to brackish-water depositional succession. The up to 6 m long cores record deposition after 11350 cal. BP. Previous studies suggest that the studied depositional succession records development common for the deglaciation phase of large, low relief, epicontinental basins. All the studied geochemical components are strongly coupled in the basal glaciolacustrine rhythmites, which reflect their rapid deposition by glacial meltwaters. Low Corg/Preac ratios suggest that a part of the rhythmite material is reworked from pre-glacial deposits. Delivery rates are drastically lower in the successive postglacial lacustrine setting due to the decrease of sedimentation rate by an order of magnitude. Decoupling of reactive Fe and associated components in the post-glacial lacustrine clays reflect increasing contribution of sediment from the emerging shoreline and rivers, but also post-depositional overprinting by sulphidization. The onset of brackish-water conditions soon after 7600 cal. BP led to 1) decoupling of reactive Fe from the detrital flux due to the establishment of salinity gradients at the river mouths, and 2) increased nutrient availability and production in water, as the intensified accumulation and burial of reactive P species, organic C and N indicate. The higher organic deposition led to deteriorated seafloor oxygen conditions; alternating thinly-laminated to strongly-bioturbated lithofacies characterise the brackish-water muds, and record temporal changes in the seafloor oxygenation. The temporal changes in the seafloor oxygen conditions appear to be broadly linked to the Holocene climate variability; brackish-water muds at the northern end of Paimionlahti Bay record anoxia during the Medieval Warm Period (1250–450 cal. BP), oxic conditions during the ‘Little Ice Age’ (450–300 cal. BP), and anoxia until the present day. On a finer scale, the redox histories in adjacent sub-basins differ considerably due to patchy topography, sluggish bottom water exchange and high organic deposition in the area. Terrestrial organic influx can temporally overprint marine organic deposition in areas close to river mouths. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The availability of P is generally considered as the main factor controlling net primary productivity in the oceans on geological time scales because, unlike nitrogen, its shortages cannot be replenished by fixation from the atmosphere (Tyrrell, 1999). Continental weathering rates and river delivery limit natural P supply to the oceans (Froelich et al., 1982; Delaney, 1998). P influx to the marine environment has varied during the Earth's glacial history (Tamburini et al., 2003). While ⁎ Corresponding author. Tel.: +358 2 3336324; fax: +358 2 3336580. E-mail addresses: joonas.virtasalo@utu.fi (J.J. Virtasalo), aarno.kotilainen@gsf.fi (A.T. Kotilainen). 0025-3227/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2008.03.008

the most complete records of glacial cycles are preserved in deep marine settings, evolution of the nutrient transport and associated processes during the last glacial cycle is recorded in higher resolution in former glaciated epicontinental basins such as the Baltic Sea. Also Fe can affect primary productivity in major ocean regions (Fung et al., 2000), as well as in the Baltic Sea (Vuorio et al., 2005). Rivers and glacial meltwaters transport significant amounts of reactive Fe to ocean margins, but this reactive Fe is substantially trapped by aggregation and deposition at estuarine salinity gradients (Poulton and Raiswell, 2002). Semi-enclosed basins such as the Baltic Sea that have experienced a pronounced lacustrine to saline water transition provide records of evolution of processes associated with the increasing trapping of reactive Fe as a response to the salinity

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increase. Reactive Fe is defined as the fraction that is present in Fe(oxyhydr)oxides, and is highly reactive toward dissolved sulphide (Canfield, 1989). Reactive Fe in both riverine and glacial meltwater environments predominantly occurs as nanoparticulate spheres associated with the edges of clay grains (Poulton and Raiswell, 2005). The Baltic Sea basin may record common trends in the late- and postglacial evolution of large, former glaciated, epicontinental basins (Virtasalo et al., 2007). The late-Pleistocene and Holocene development of the basin is a succession of glaciolacustrine, post-glacial lacustrine and brackish-water depositional environments (Virtasalo et al., 2007). The onset of brackish-water conditions resulted from the mid-Holocene incursion of the Atlantic into the basin (Winterhalter et al., 1981). For the past 4000 years, Baltic Sea has been affected by the North Atlantic climate that controls the freshwater runoff and irregular pulses of ocean water into the basin (Hänninen et al., 2000; Burke and Kemp, 2004). Deep areas of the Baltic Sea predominantly are anoxic due to the permanent salinity stratification of the water mass. However, complex topography, seasonal thermal stratification, sluggish water exchange and high organic deposition rate cause anoxia also in shallow coastal areas in the northern parts of the basin (Virtasalo et al., 2005a). In this paper, the geochemistry of sedimentary P species, reactive Fe-oxide-associated Fe (and Al and Mn), C, N and S is studied in sediment cores from three sites in the Archipelago Sea in the northern Baltic Sea. The purpose is to gain understanding on the factors controlling P species and reactive Fe behaviour in the depositional succession as a response to the ice margin retreat and the establishment of brackish-water conditions. The results are discussed in comparison to other areas of the Baltic Sea basin, and in relation to the Holocene climate variability. This study will contribute toward understanding the processes associated with the late- and postglacial flux of P and reactive Fe into the oceans.

2. Regional setting The Archipelago Sea in northern Baltic Sea (Fig. 1) is an expansive mosaic of more than 22 000 islands within a surface area of almost 8000 km2. The average water depth is only 23 m, but some deeps reach over 100 m. The waters are annually covered by ice for 3– 4.5 months, the ice-bound period usually ending in April. The sea is seasonally stratified; thermal stratification develops during early summer, and the water mass is completely mixed in September (Mälkki et al., 1979). Eight rivers drain into the sea area. The largest river, the Paimionjoki river (mean suspended load 3.03 × 106 kg a− 1, Mansikkaniemi, 1974), is located closest to the sampling sites. The river has a seasonal freshwater lens with increased turbidity and nutrients. The freshwater lens only occasionally reaches the study site (AS5) closest to the river mouth. There is no significant productivity gradient between the sampling sites; the surface mean dissolved P ranges from 24 to 29 μg dm− 3 (Suomela and Sydänoja, 2006). Surface salinity ranges from 5 to 7 PSU from the inner archipelago toward the open sea. The sea is essentially tideless, but irregular water level fluctuations of up to 1.3 m occur as a result of variations in wind and atmospheric pressure. The shallow water and abundant islands prevent substantial water exchange between the archipelago and the open sea areas (Mälkki et al., 1979). The sluggish circulation, seasonal stratification of the water mass and high accumulation and degradation of organic matter result in strong seafloor oxygen gradients (Virtasalo et al., 2005a). Sedimentation patterns are complicated by glacio-isostatic uplift (3–4 mm a− 1, Mäkinen and Saaranen, 1998), which results in slowly shifting boundaries between the areas of sediment accumulation, transportation and erosion. The deglaciation of the Archipelago Sea area began when the Fennoscandian continental ice sheet retreated from the Second Salpausselkä ice-marginal accumulation in the SE part of the area

Fig. 1. Map of the study area and sampling sites. Second Salpausselkä (Ss II) is a continuous ice-proximal subaquatic glaciofluvial ridge deposited during a halt in the ice front retreat.

J.J. Virtasalo, A.T. Kotilainen / Marine Geology 252 (2008) 1–12

(Fig. 1) at 11590 ± 110 calendar years before present (cal. BP) (Saarnisto and Saarinen, 2001). The subaqueous ice margin retreated toward the NNW, and the rest of the area was deglaciated during the subsequent 700 clay-varve years (Strömberg, 2005). The retreating ice front left behind till and outwash sands and gravels, which partly cover the underlying crystalline bedrock. The glaciolacustrine varved silts and clays of the Dragsfjärd Alloformation overlay this substratum (Fig. 2; Virtasalo et al., 2005b). The varved deposits are followed by the Korppoo Alloformation, which comprises two units: 1) the Trollskär Allomember consisting of reworked varved clasts in a clay–silt matrix and 2) the Sandön Allomember composed of post-glacial lacustrine weakly-layered to strongly-bioturbated clays. The superimposed Nauvo Alloformation is composed of brackish-water, organic-rich muds characterised by alternating thinly-laminated, weakly-laminated and strongly-bioturbated lithofacies. The deposition of these muds in the northern Baltic Sea basin began soon after 7600 cal. BP due to the incursion of the Atlantic through the sounds in Kattegat caused by the mid-Holocene glacio-eustatic ocean-level rise (Virtasalo et al., 2006, 2007). The thinly-laminated facies record seafloor anoxia, while the more bioturbated intervals reflect more or less aerobic depositional conditions (Morris et al., 1988; Virtasalo et al., 2005a, 2006). 3. Materials and methods 3.1. Fieldwork Four sediment cores were collected onboard the Finnish R/V Aranda in September 2003 (Fig. 1; Table 1). Long piston coring equipment was used, and the coring locations were selected on the basis of acoustic profiles. The cores were retrieved from the sub-basin flanks, where older deposits were not buried too deep for the coring equipment. The cores AS5-PC1 and AS5-PC2 are replicate cores collected at the same location. Another criterion for selecting the coring locations was to sample seafloor areas with different redox conditions; CTD profiles with oxygen recordings of the water column were measured at every station prior to coring (Aranda Cruise Report 12/2003, Finnish Institute of Marine Research). The coring locations of AS2-PC4 and AS3-PC1 represented oxic near-bottom water conditions

Fig. 2. Rough correlation of the Archipelago Sea Holocene stratigraphy (Virtasalo et al., 2005b) at the AS2-PC4 coring site to the Baltic Sea palaeoenvironmental phases (Winterhalter et al., 1981). The dates are from Virtasalo et al. (2007). Dashed lines indicate subtle environmental changes. Salinity information is from sedimentary diatom studies (Heinsalu, 2001; Tuovinen et al., in press).

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Table 1 Coring location, water depth and recovery of the studied sediment cores Core

Sampling date

Latitude (WGS84)

Longitude (WGS84)

Water depth (m)

Recovery (m)

AS2-PC4 AS5-PC1a AS5-PC2a AS3-PC1

23 25 25 26

60°02.60N 60°18.47N 60°18.47N 60°13.27N

22°17.71E 22°30.03E 22°30.03E 22°25.98E

32 33 33 19

5.97 5.60 5.56 4.23

a

Sep. 2003 Sep. 2003 Sep. 2003 Sep. 2003

Replicate cores.

(O2 N4 ml dm− 3), while the oxygen conditions at the AS5-PC2 site were poor (O2 b2 ml dm− 3). The cores were up to 6 m long and they were cut into ~150 cm long sections for transportation. AS2-PC4 was opened and sub-sampled onboard. The other cores were stored in a refrigerator, and opened and studied ashore. 3.2. Laboratory analysis The core sections were slabbed and trimmed for lithologic description and digital photographing. Bulk magnetic susceptibility was measured at 5 mm intervals using a Bartington MS2E1 Surface Scanning Sensor. Sub-samples for geochemical analysis were taken with an open-tip syringe, and immediately put in a deep freezer. The core AS2-PC4 was sub-sampled for palaeomagnetic studies using cubic polystyrene sample boxes removed at every ~2.5 cm. Shell and fish bone material, and marine plant fragments were collected for AMS-14C dating. Sub-samples (volume of 2 cm3) for dry bulk density (DBD) and weight loss on ignition (LOI) analyses were collected; the values were determined after drying the samples at 105 °C for 12 h and ashing at 550 °C for 2.5 h, respectively. Grain size distributions were analysed using a Micromeritics 5000 ET Sedigraph. Sediment slabs 50 × 4 × 1 cm in cross-section of the entire cores were X-rayed using Philips constant potential 102 L equipment and AGFA Structurix D7Pb film. All the geochemical and physical analyses were performed for the cores AS2-PC4 and AS3-PC1. For AS5-PC2, only LOI, grain size distributions and C, N and S concentrations were determined. Its replicate core AS5-PC1 was stored frozen, and analysed for LOI, P species and Fe-oxide-associated Fe, Mn and Al, together with several other geochemical parameters (manuscript in preparation). AS5-PC1 and AS5-PC2 were correlated by comparing their lithology and LOI data. Total carbon (Ctot), N and S were analysed using an Elementar vario MAX CN instrument and a Leco SC132 Sulfur determinator at the Geological Survey of Finland. For the organic carbon (Corg) determination, the samples were pretreated with a direct acidification method (Ryba and Burges, 2002), and analysed for C concentrations using a Leco CNS-2000 instrument at the Pirkanmaa Regional Environment Centre (standard SFS-EN 13137, method B). Carbonate carbon (Ccarb) concentrations were calculated as the difference between Ctot and Corg. Sedimentary phosphorus speciation for 201 samples was obtained using the sequential extraction technique developed by Ruttenberg (1992) as modified by Anderson and Delaney (2000). The modified procedure omits the first step of the original procedure that extracts the exchangeable fraction of P; the concentration of exchangeable P is usually b10% of P extracted in the second step of the original procedure in these sediments (Kaarina Lukkari, pers. comm.). The applied procedure chemically extracts four operationally defined fractions: 1) solid phase-associated P (PCDB), adsorbed onto or coprecipitated with solid phases such as Fe-(oxyhydr)oxides, 2) authigenic and biogenic P (Paut), associated with authigenic P-rich minerals and biogenic debris, 3) detrital P (Pdet), assumed to represent detrital apatite, and 4) organic P (Porg), associated with residual organic matter. The extraction procedure follows that applied in Virtasalo et al. (2005a). In the first step, the samples were treated with a solution

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Fig. 3. Stratigraphy, depositional environments (after Virtasalo et al., 2007), lithology, grain size median, magnetic susceptibility (κ) and weight loss on ignition (LOI, in % dry weight) for the sediment cores. Dashed lines indicate the lithologic core-to-core correlations of stratigraphic units. Thin dashed lines indicate lithofacies correlations between the replicate cores AS5-PC1 and AS5-PC2. Zigzag lines indicate erosional boundaries of the stratigraphic units. Weakly-layered (WL), monosulphide-banded (MB), monosulphide-mottled (MM) and bluish-grey (BG) lithofacies are indicated for the post-glacial lacustrine clays (Sandön Allomember). Strongly-bioturbated (B), weakly-laminated (W) and thinly-laminated (L) lithofacies are indicated for the brackish-water muds (Nauvo Alloformation). Asterisks indicate AMS-14C sub-sample collection depths for AS5-PC1 (see Table 2).

consisting of 0.22 M Na-citrate, 0.033 M Na-dithionite and 1.0 M Nabicarbonate for 6 h (pH 7.6), followed by subsequent treatments of 1 M MgCl2 and H2O for 2 h each. In the second step, the residues were treated with 1 M Na-acetate (buffered to pH 4 with acetic acid) for 5 h, followed by one treatment of 1 M MgCl2 for 2 h, one treatment of 1 M MgCl2 for 1 h and one treatment of H2O for 1 h. In the third step, the residues were treated with 1 M HCl for 16 h. In the fourth and final step, the residues were dried at 105 °C for 12 h with a 50% (w/v) MgNO3 solution, ashed at 550 °C for 2.5 h, then treated with 1 M HCl for 16 h. The samples were analysed in duplicate in a series of 30 samples including blank samples. Phosphate concentrations were determined using a standard ascorbic acid molybdate blue technique (Murphy and Riley, 1962), except the supernatants from the first step. Sample absorbances were measured using a HACH DR/2000 spectrophotometer at 885 nm. Reactive phosphorus (Preac) concentrations were calculated as the sum of PCDB, Paut and Porg. Detection limits for Paut, Pdet and Porg are 0.5 μmol g− 1. Mean standard deviations of three replicates indicate 8.2% error for Paut, 6.8% error for Pdet and 7.6% error for Porg extractions. P concentrations, together with Fe, Al and Mn, in the first step supernatants were measured using an ELAN 6000 ICP-MS at the Laboratory of Analytical Chemistry of Åbo Akademi University. FeCDB extracted in this step is assumed to be a measure of ‘reactive’ Fe, bound in Fe-(oxyhydr)oxides, and in amorphous Fe sulphides (Ruttenberg, 1992; Slomp et al., 1996). AlCDB and MnCDB extracted in this step have not been calibrated using reference materials, but they are thought to be measures of Al and Mn in (oxyhydr)oxides leaving carbonates largely intact (Ruttenberg, 1992). MnCDB also includes all the forms of Mn(III) or Mn(IV) (oxyhydr)oxides, which are all easily

reducible (Anschutz et al., 2005). Detection limits for PCDB, AlCDB, MnCDB and FeCDB are 0.3 μmol g− 1. Mean standard deviations of three replicates indicate 8.1% error for PCDB, 8.0% error for AlCDB, 7.4% error for MnCDB and 6.3% error for FeCDB extractions. 3.3. Sediment chronology and accumulation rates Dating of AS2-PC4 is described in Virtasalo et al. (2007). Briefly, the sample cubes collected from the core were analysed for natural remanent magnetization (NRM) declination and inclination using a 2G Enterprises SRM-755R tri-axial SQUID magnetometer. A sediment chronology was constructed by matching the NRM direction patterns against accurately dated reference curves measured for the annually laminated Lake Nautajärvi sediments from southern Finland (Ojala and Saarinen, 2002). The Lake Nautajärvi reference chronology covers the past 10 000 years, with an overall dating error of ±1%. The same dating error is assumed for the AS2-PC4 sediment chronology. Table 2 AMS-14C dates of the available macrofossils in the core AS5-PC1 Laboratory number

Core depth (cm)

LuS 6530 LuS 6529

40 97

LuS 6528

276.5

14

C-age BP

102.2 ± 0.5 pMC 280 ± 50 1715 ± 50

Calibrated age range BP (95.4% probability) 224–253a 272–485a 1269–1500b

Sample material Shell Marine plant fragment Marine plant fragment

pMC = percent modern carbon. a Calibrated with Intcal04 dataset (Reimer et al., 2004) due to their young age. b Calibrated with Marine04 dataset (Hughen et al., 2004).

45.3 (16.0) 45.3 (15.4) 39.3 (12.8) 30.5 (11.8) 24.7 (2.0) 21.0 (6.3) 78.8 (28.6) 79.2 (29.9) 63.5 (22.0) 42.5 (14.9) 37.8 (4.3) 37.9 (12.9) 294 (234) 59.4 (163) 30.0 (39.1) 13.7 (11.8) 17.2 (9.7) 15.6 (9.0) 137 (27.0) 113 (11.3) 109 (7.4) 100 (10.0) 85.1 (6.2) 84.1 (16.3) 42.6 (67.2) 11.7 (19.0) 12.3 (17.3) 30.3 (22.5) 47.9 (36.8) 9.5 (11.2) 803 (304) 642 (96.5) 530 (55.8) 431 (46.9) 314 (21.9) 374 (107) 4.1 (3.9) 18.8 (9.8) 21.7 (18.0) 27.4 (7.8) 19.4 (5.8) 28.7 (11.2) 1.1 (0.34) 0.54 (0.28) 0.55 (0.38) 0.78 (0.29) 0.85 (0.17) 1.1 (0.31) (2.6) (1.2) (1.2) (1.6) (1.1) (2.3) 6.2 5.6 5.2 9.2 6.3 9.3 34.9 (1.7) 31.8 (4.5) 33.4 (3.6) 35.9 (3.4) 36.1 (1.5) 39.7 (3.0) 10.3 (1.9) 8.4 (3.1) 8.6 (2.6) 9.9 (2.3) 8.6 (1.3) 11.2 (3.0) 17.1 (2.3) 17.3 (1.7) 19.8 (1.6) 21.2 (2.3) 22.8 (1.9) 20.5 (2.5) n.d. = value not detected. Standard deviations are given in brackets. a Calculated as the sum of PCDB, Paut, Pdet and Porg. b Calculated as the difference between Ctot and Corg. c Calculated as the sum of PCDB, Paut and Porg.

2.9 3.4 2.4 2.6 2.2 4.7

(0.7) (1.4) (1.3) (1.7) (0.97) (1.5)

4.6 2.6 2.6 2.2 2.5 3.4 11 35 24 9 5 20 Post-glacial lacustrine clays (Sandön Allomember) Bluish-grey Monosulphide-mottled Monosulphide-banded Weakly-layered Debris-flow deposits (Trollskär Allomember) Glaciolacustrine rhythmites (Dragsfjärd Alloformation)

(0.52) (0.92) (0.48) (0.40) (0.23) (0.71)

51.4 (19.0) 55.8 (34.8) 88.0 (32.3) 70.0 (8.8) 103 (42.4) 145 (39.7) 158 (23.6) 142 (89.3) 200 (141) n.d. 253 (8.0) 48.1 (64.3) 270 (22.6) 17.7 (32.2) 300 (47.6) 92.8 (20.7) 1887 (43.0) 66.6 (32.0) 1964 (175) 55.8 (50.2) 2237 (389) 3.7 (2.4) 2.7 (1.8) 2.3 (1.7) 62.7 (21.3) 10.4 (2.4) 62.7 (20.4) 11.3 (5.8) 44.7 (9.7) 10.5 (5.0) 27.0 (2.8) 21.6 (7.3) 16.1 (2.8) 2.7 (0.79) 2.6 (1.1) 3.1 (2.0) 7 12.1 (17.4) 23 19.9 (15.5) 18 8.3 (8.3)

20.9 (1.7) 19.0 (4.9) 17.3 (6.5)

95.3 (6.2) 99.1 (19.7) 140 (15.5) 142 (28.4) 150 (22.1) 354 (80.3) n.d. 338 (23.1) 33.5 (65.8) 350 (72.0) 2456 (159) 2596 (613) 1.8 (0.23) 74.0 (30.1) 0.90 (0.39) 2.8 (1.4) 10.2 (1.7) 5.6 (1.5) 43.6 (1.8) 43.6 (4.4) 17.6 (0.79) 18.5 (4.0) 2.2 (0.15) 4.7 (0.75) 4 6.0 (1.6) 38 3.3 (1.3)

17.8 (0.56) 17.2 (1.3)

87.2 (17.5) 153 (18.2) 317 (80.7) 2164 (1024) 27.8 (54.6) 309 (130) 0.73 (0.27) 1.4 (0.60) 2.5 (1.2) 38.1 (4.2) 13.7 (5.7) 14.7 (4.5) 7.4 (1.8) 2.3 (1.1)

The lithology and physical properties of studied cores are described in detail in Virtasalo et al. (2005b); they are briefly reintroduced below. Also, a chronology for AS2-PC4 is presented in Virtasalo et al. (2007). The replicate cores AS5-PC1 and AS5-PC2 are similar (Fig. 3); their results are pooled and discussed as AS5-PC2. For discussion on the depositional environments of the sediment units, see Virtasalo et al. (2007). The basal part of AS2-PC4 consists of glaciolacustrine rhythmites (Dragsfjärd Alloformation, Fig. 3) deposited between 11350 and 11100 cal. BP. Magnetic susceptibility values are high in the sediments, and increase with the grain size, while LOI values are low and decrease with the increasing grain size. The monosulphide-mottled interval at 547–538 cm (Fig. 3) resembles the interval associated with the shortlived brackish-water influence in the northern Baltic Sea basin during 11300–11200 cal. BP (Heinsalu, 2001). Sedimentation rate estimated for these deposits based on the mean varve thickness is 13 mm a− 1. The rhythmites are partly and unconformably overlain by a unit composed of rhythmite clasts in a clay–silt matrix (Trollskär Allomember, Fig. 3). This unit is a debris-flow deposit partly composed of material reworked from the underlying rhythmites. The unit has been excluded from statistical analysis due to its mixed origin. Post-glacial lacustrine clays (Sandön Allomember, Fig. 3) overlie the rhythmites and debris-flow deposits. They were deposited during 10 300–7900 cal. BP in AS2-PC4. The unit is a gradational lithofacies

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4.1. Core lithology and sediment chronology

n

4. Results and interpretation

Table 3 Mean molar geochemical concentrations and Corg/Porg and Corg/Preac ratios for the sediment units

Geochemical concentrations below detection were rounded to half the detection limits, so that approximate values could be used. In order to objectively describe the relationships and distribution of the analysed geochemical components, a principal component analysis (PCA) was carried out using the SPSS 11 software. The data set was split at the major stratigraphic boundaries and the sediment units were analysed separately. The brackish-water muds (Nauvo Alloformation) were further divided into thinly-laminated, weakly-laminated and strongly-bioturbated lithofacies. PCAs were performed on Spearman rank correlation matrixes because the raw data did not meet the normality of distribution assumption. PCA factors exceeding the eigenvalue of 1 were selected for interpretation and given a Varimax rotation. Detailed relationships between the analysed geochemical components within each stratigraphic unit were derived from linear regression models except for FeCDB, PCDB, MnCDB and AlCDB, which were derived from linear reduced major axis (RMA) regression models that are appropriate when there is no a priori functional relation of dependency between the variables tested. The RMA models were calculated using the SMATR 2 freeware (Falster et al., 2006) and plotted using the MATLAB 6.5 software.

Sediment unit

3.4. Statistical analysis

PCDB Paut Pdet Porg Ptota AlCDB MnCDB FeCDB Corg (µmol g− 1) (µmol g− 1) (µmol g− 1) (µmol g− 1) (µmol g− 1) (µmol g− 1) (µmol g− 1) (µmol g− 1) (µmol g− 1)

Ccarbb N S Corg/Porg (μmol g− 1) (μmol g− 1) (μmol g− 1)

Radiocarbon analyses for available macrofossils in the cores were made at the Radiocarbon Dating Laboratory of Lund University in Sweden. The AMS-14C dates were calibrated to calendar years (0 cal. BP = 1950) using the CALIB REV5.0.2 software. The values were corrected for the reservoir effect by applying the Marine04 calibration dataset with the Baltic Sea regional average ΔR value of −107 ± 24 (Hughen et al., 2004). For ages younger than 300 radiocarbon-years, the Intcal04 calibration dataset was used (Reimer et al., 2004). Geochemical component accumulation rates (CAR, in μmol cm− 2 a− 1) were calculated by: CAR=z /t × DBD ×C, where z is the thickness of a sediment segment (cm), t is the accumulation time of the sediment segment (a), DBD is the dry bulk density of the sediment (g cm− 3) and C is the concentration of a certain geochemical component in the sediment (μmol g− 1).

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Brackish-water muds (Nauvo Alloformation) AS2-PC4 Strongly-bioturbated AS3-PC1 Weakly-laminated Strongly-bioturbated AS5-PC2 Thinly-laminated Weakly-laminated Strongly-bioturbated

Corg/Preacc

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succession comprising 1) weakly-layered to homogeneous facies, 2) monosulphide-banded homogeneous facies, 3) monosulphidemottled homogeneous facies, and 4) bluish-grey homogeneous facies. The monosulphide banding and mottling is diagenetic (Boesen and Postma, 1988; Sternbeck and Sohlenius, 1997; Böttcher and Lepland, 2000), and may not represent primary depositional processes. Susceptibility values decrease at the allomember base, remain relatively constant upward, and decrease again at the base of the bluish-grey facies. LOI values increase at the allomember base, remain relatively constant upward, and increase again in the bluish-grey facies. An erosional unconformity bounds the allomember at the top. The bluish-grey facies is missing in AS5-PC2 due to this erosion (Fig. 3). Linear sedimentation rate of the post-glacial lacustrine clays in AS2-PC4 is 1.1 mm a− 1. The same sedimentation rate is assumed for these clays in AS3-PC1 and AS5-PC2 because no macrofossils suitable for AMS-14C dating were found in these deposits in these cores. Superimposed to the aforementioned deposits are the brackishwater, organic-rich muds of the Nauvo Alloformation (Fig. 3). One or more 1–2-cm-thick layers of silt–fine sand separate the brackishwater muds from the underlying deposits. This contact corresponds to the Holocene lacustrine to brackish-water transition of the Baltic Sea basin, or the so-called Ancylus/Littorina boundary (e.g. Winterhalter et al., 1981). There is a hiatus between the post-glacial lacustrine and

brackish-water deposition in the northern Baltic Sea basin (Virtasalo et al., 2007). The deposition of brackish-water muds began in AS2-PC4 at approximately 4300 cal. BP (Virtasalo et al., 2007). In AS5-PC1, the age of brackish-water mud section base is extrapolated to 1500 cal. BP based on the available AMS-14C dates (Fig. 3; Table 2). Age–depth relationship in AS5-PC1 is linear, and the calculated linear sedimentation rate of 2.0 mm a− 1 fits within the calibrated 95.4% probability age ranges for the core (Appendix A). The linear sedimentation rate is interpolated on the basis of the mean value of the oldest available calibrated age range, which was used for the calculation because it could be reliably calibrated using the Marine04 dataset with correction for the regional reservoir effect, while the other radiocarbon ages were too young for the Marine04 dataset. The interpolated sedimentation rate approximates the combined thickness of the dark–light lamina couplet layers in these deposits (see below). No AMS-14C dates were determined for AS3-PC1, but, as the brackish-water mud section of that core is of comparable length to that of AS5-PC2 (Fig. 3), it is reasonable to assume a similar deposition rate for these muds in both cores. Analysis of X-ray images for AS5-PC2 and AS3-PC1 shows no major breaks in sedimentation. The brackish-water mud section is shortest in AS2-PC4, where it is almost structureless with heavy bioturbation (Fig. 3). In AS3-PC1, the mud section is strongly-bioturbated, as well, except for the weakly-

Fig. 4. Molar geochemical component accumulation rates (CARs, in μmol cm− 2 a− 1) and Corg/Porg and Corg/Preac ratios for the sediment cores. The sediment units are indicated in the black vertical bars on the left: 1 = glaciolacustrine rhythmites (Dragsfjärd Alloformation) and debris-flow deposits (Trollskär Allomember), 2 = post-glacial lacustrine clays (Sandön Allomember), 3 = brackish-water muds (Nauvo Alloformation). Dashed line indicates the unconformity separating the glaciolacustrine rhythmites and post-glacial lacustrine clays. Black horizontal bars indicate the hiatus between the post-glacial lacustrine clays and brackish-water muds. Vertical axes break at the hiatus. For the brackish-water muds, the thinlylaminated lithofacies are indicated with dark shading, the weakly-laminated lithofacies are indicated with light shading, while the strongly-bioturbated lithofacies are not shaded. Note that for AS2-PC4, the CARs are plotted on different horizontal axes for the brackish-water muds than for the underlying deposits in order to display the high values for the glaciolacustrine rhythmites.

J.J. Virtasalo, A.T. Kotilainen / Marine Geology 252 (2008) 1–12

7

laminated topmost 33-cm interval. In AS5-PC2, the mud section is characterised by alternating thinly-laminated, weakly-laminated and strongly-bioturbated lithofacies. The thinly-laminated facies represent the annual layering of dark couplet layers consisting of algae and lipid-rich organic matter, which correspond to periods of high (vernal) phytoplankton production, and light clastic couplet layers corresponding to (winter) times of low production (Morris et al., 1988). Both couplet layers are on average 1 mm thick. These thinly-laminated facies are interpreted to be indicative of seafloor anoxia with no bioturbation, while the strongly-bioturbated facies represent aerobic seafloor conditions. The weakly-laminated facies, in turn, are characterised by lighter and darker sub-intervals and occasional bundles of few thin laminae. The weakly-laminated facies are taken to reflect supra-annual, irregular, redox fluctuations at the sediment– water interface (Virtasalo et al., 2005a). The colour of dark subintervals is indicative of ferrous monosulphides formed during oxygen depletion, while the light sub-intervals are coloured by ferric (oxy) hydroxides precipitated under oxic conditions (Mortimer, 1941). The occasional thin-lamina bundles in the weakly-laminated facies are remnants of the primary laminated structure that has been almost completely disturbed by successive, temporarily permitted, bioturbation. Susceptibility values are low in the brackish-water muds, and decrease upward. LOI values are high and increase upward. 4.2. Geochemistry Mean geochemical concentrations in the sediment units (Table 3) are similar to those reported earlier for the Archipelago Sea area (Gripenberg, 1934; Rantataro, 1999; Virtasalo et al., 2005a). Because of marked differences in the deposition rates between the sediment units, we not only report our data as accumulation rates (CARs: see Fig. 5), but also as elemental ratios (see also Anderson and Winckler, 2005). The molar concentrations at each depth are listed in Appendix B. Supplementary materials I to III. 4.2.1. Glaciolacustrine rhythmites (Dragsfjärd Alloformation) High CARs characterise the glaciolacustrine rhythmites (Fig. 4), which is mainly a consequence of rapid deposition. The geochemical concentrations do not differ to such a large degree from the other sediment units; only Corg, N and S are markedly lower in the rhythmites (Table 3). A positive relationship between FeCDB and AlCDB, MnCDB and PCDB (Table 4; Fig. 5a) indicates that the majority of AlCDB, MnCDB and PCDB Table 4 Results of linear reduced major axis (RMA) regression analyses carried out for selected geochemical components in the sediment units n Dragsfjärd Alloformation AlCDB vs. FeCDB 20 MnCDB vs. FeCDB 20 PCDB vs. FeCDB 20 Nauvo Alloformation Thinly-laminated AlCDB vs. FeCDB 7 MnCDB vs. FeCDB 7 PCDB vs. FeCDB 7 Weakly-laminated AlCDB vs. FeCDB 23 MnCDB vs. FeCDB 23 PCDB vs. FeCDB 23 Strongly-bioturbated AlCDB vs. FeCDB 66 MnCDB vs. FeCDB 66 PCDB vs. FeCDB 66 Nauvo Alloformation in AS5-PC2 Paut vs. Pdet 35 Porg vs. Pdet 35 See Fig. 5 for selected plots.

Slope

Intercept

p

r2

0.26 0.03 0.13

1.7 0.34 0.86

b 0.001 0.001 0.001

0.67 0.46 0.48

0.11 0.11 0.84

−0.17 −7.0 −65.9

0.004 0.001 0.001

0.84 0.90 0.91

0.17 0.05 0.51

−0.42 −1.0 −17.1

0.045 b 0.001 0.002

0.18 0.51 0.36

0.11 0.03 0.14

4.5 0.69 2.1

b 0.001 b 0.001 b 0.001

0.71 0.85 0.58

0.29 2.0

−2.8 −14.7

b 0.001 b 0.001

0.53 0.66

Fig. 5. Selected plots of relationships between geochemical components in the sediment units. Coefficients of these RMA regression results are presented in Table 4. (a) Relationship between FeCDB and PCDB for the glaciolacustrine rhythmites (Dragsfjärd Alloformation). (b) Relationships between FeCDB and PCDB for the thinly-laminated (squares, dash-dot line), weakly-laminated (triangles, solid line) and stronglybioturbated (circles, dashed line) lithofacies of the brackish-water muds (Nauvo Alloformation). (c) Relationship between Pdet and Porg for the brackish-water muds (Nauvo Alloformation) in the core AS5-PC2.

occur in close association with Fe-(oxyhydr)oxides in the rhythmites. The y-intercepts of the AlCDB/FeCDB, MnCDB/FeCDB and PCDB/FeCDB regression lines have small positive values (Table 4), suggesting that when Fe-(oxyhydr)oxides are lacking in the sediments, only small amounts of AlCDB, MnCDB or PCDB are present; the residues probably represent ‘extra’ solid phase-associated Al, Mn and P bound loosely in these deposits, and extracted along this first step of sequential extraction procedure. PCA indicates that practically all the variance of the concentrations of geochemical components can be explained by a single factor (Table 5), which indicates very similar behaviour for the geochemical components

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Table 5 Principal component factor loadings for the glaciolacustrine rhythmites (Dragsfjärd Alloformation) Factor 1 PCDB Paut Pdet Porg FeCDB MnCDB AlCDB Corg N S

0.99 0.87 −0.99 0.99 0.98 0.99 0.99 0.76 0.97 −0.94

Total variance explained: 90.2%. The factor matrix is not rotated because only one factor was extracted.

in the rhythmites. Negative loadings for Pdet and S contrast with the rest of the geochemical components, which is taken to reflect properties in the source material. Pdet is the most abundant P-form, comprising on average 52.0% of total P. Of the reactive P phases (PCDB, Paut and Porg), Porg is the most abundant and makes up on average 27.7% of total P. A positive relationship between Corg and Ctot is strong (Corg = 0.03 + 0.91 × Ctot, r2 = 0.98, p b 0.001).

positive loadings for PCDB, Corg and N, but a negative loading for Pdet; this factor describes the accumulation of primary organic matter, P adsorbtion to sediment particles, and dilution by detrital matter. Factor 3 has high loadings for Porg and MnCDB, but also for Paut and N; this factor reflects the delivery of refractory organic P to the deposits, and its subsequent transformation to authigenic phosphates (Nriagu and Dell, 1974; Emerson and Widmer, 1978; Froelich et al., 1982; Delaney, 1998). Altogether, FeCDB, AlCDB, MnCDB and PCDB are dominantly loaded on different PCA factors, reflecting poor coupling between these presumably Fe-(oxyhydr)oxideassociated geochemical components. Similarly the regression analysis indicates statistically insignificant relationships between FeCDB, AlCDB, MnCDB and PCDB, which denotes that AlCDB, MnCDB and PCDB are not only associated with Fe-(oxyhydr)oxides in these deposits. It can be that a larger portion of FeCDB, AlCDB, MnCDB and PCDB are adsorbed to clay particles in the post-glacial clays compared to the other deposits. A positive relationship between Corg and Ctot is strong (Corg = 0.046 + 0.93 × Ctot, r2 = 0.84, p b 0.001). Pdet is the dominating P-form, making up on average 56.7% of total P. Porg is the most abundant reactive P-form; it makes up 26.3% of total P.

4.2.2. Post-glacial lacustrine clays (Sandön Allomember) Post-glacial lacustrine clays have significantly lower delivery rates of studied geochemical components compared to the glaciolacustrine rhythmites (Fig. 4), which is due to the dramatically decreased sedimentation rate. PCDB, Paut and MnCDB contents in the basal weaklylayered lithofacies are lower than in the underlying rhythmites, while the contents of other geochemical components are similar to the rhythmites (Table 3). Pdet, AlCDB and FeCDB contents decrease upward, while Paut, Corg, N and S contents increase upward the post-glacial lacustrine facies succession. Especially FeCDB is depleted and S is enriched in the top bluish-grey facies. Differences in the geochemical composition of facies between the cores show no clear lateral tends, which probably reflects the mixed origin of sediments from various distant sources such as meltwaters at the ice front, reworking of underlying strata at shallow areas, and river load. The behaviour of geochemical components in the post-glacial lacustrine clays can be explained by three PCA factors (Table 6). Factor 1 shows strong positive loadings for S and Paut, but negative loadings for FeCDB and AlCDB; this factor is taken to represent the dissolution of Fe(oxyhydr)oxides and the authigenic precipitation of sulphides and P-rich minerals (Froelich et al., 1982; Delaney, 1998). These sediments contain very little carbonate (Table 3), which suggests insignificant amounts of CaCO3-bound P. The dominant authigenic P-rich minerals are phosphates such as apatite and vivianite, which readily form in lake sediments (Nriagu and Dell, 1974; Emerson and Widmer, 1978). Factor 2 shows strong

4.2.3. Brackish-water organic-rich muds (Nauvo Alloformation) The delivery rates of geochemical components are slightly higher in the brackish-water muds compared to the post-glacial lacustrine clays (Fig. 4), except the delivery rates of Corg, N and S that are markedly higher than those of the post-glacial lacustrine clays. Contents of the geochemical components are different between the lithofacies of brackish-water muds (Table 3). Reactive Fe and associated components are enriched in the weakly-laminated facies, while authigenic P and S are enriched in the strongly-bioturbated facies. Geochemical concentrations also differ between the cores (Table 3). Detrital P contents decrease from AS5-PC2 to AS3-PC1 to AS2-PC4, i.e. with increasing distance to the Paimionjoki river (Fig.1). Similarly, contents of reactive Fe and associated components decrease in the cores with increasing distance to the river. Porg, Corg, and N contents are similar between the facies and the cores, except for the laminated sections in AS5-PC2 that are poor in Corg and N, and abundant in Porg. Two PCA factors can explain the behaviour of geochemical components in these muds (Table 7). Factor 1 shows strong loadings for PCDB, Paut, Porg, FeCDB, MnCDB, AlCDB and S. These geochemical components are associated with the redox-dependent dissolution of Fe-(oxyhydr)oxides, the release of associated components PCDB, MnCDB and AlCDB, and subsequent authigenesis of P-rich apatite (Froelich et al., 1982; Delaney, 1998). Paut and S have loadings opposite to the other components, which reflects the dissolution and degradation during the remineralization process, and the authigenic precipitation of minerals such as apatite and sulphides. Factor 1, thus, represents redox-driven remineralization in the sediments. Factor 2 shows strong positive loadings for Corg and N, but a strong negative loading for Pdet; this factor reflects primary organic accumulation and dilution by detrital matter.

Table 6 Varimax rotated principal component factor loadings for the post-glacial lacustrine clays (Sandön Allomember)

Table 7 Varimax rotated principal component factor loadings for the brackish-water muds (Nauvo Alloformation)

Factor 1 PCDB Paut Pdet Porg FeCDB MnCDB AlCDB Corg N S

−0.31 0.70 −0.28 −0.87 −0.88 0.56 0.50 0.80

Factor 2 0.84 −0.91

Factor 3 0.27 0.60 −0.27 0.97

−0.31 0.31

0.92

0.80 0.70 0.34

0.50 0.36

Factor 1 (remineralization) explains 34.0%, Factor 2 (primary organic deposition) 30.5% and Factor 3 (reworked organic P deposition) 26.8% of the variance. Total variance explained: 91.4%. Loadings between − 0.25 and 0.25 are not shown.

Factor 1 PCDB Paut Pdet Porg FeCDB MnCDB AlCDB Corg N S

0.96 −0.90 0.59 0.91 0.95 0.96 −0.61 −0.62 −0.90

Factor 2 0.36 −0.95 −0.38 −0.27 0.76 0.75 0.37

Factor 1 (remineralization) explains 63.2% and Factor 2 (primary organic deposition) 26.1% of the variance. Total variance explained: 89.2%. Loadings between −0.25 and 0.25 are not shown.

J.J. Virtasalo, A.T. Kotilainen / Marine Geology 252 (2008) 1–12

A significant positive relationship between FeCDB and AlCDB, MnCDB and PCDB in the strongly-bioturbated facies of brackish-water muds (Table 4; Fig. 5b) indicates that the majority of AlCDB, MnCDB and PCDB occur in close association with Fe-(oxyhydr)oxides in the sediments deposited under oxic seafloor conditions. However, the y-intercept values of the PCDB/FeCDB, MnCDB/FeCDB and AlCDB/FeCDB regression lines decrease with bioturbation of sediment (Table 4). Porg and Pdet are nearly equally abundant in these muds, making up on average 39.1% and 38.2% of total P, respectively. A positive relationship between Corg and Ctot is strong (Corg = 0.023 + 0.1 × Ctot, r2 = 0.97, p b 0.001). Strong coupling between Corg and N (Corg = −0.16 + 7.88 × N, r2 = 0.94, p b 0.001) indicates that practically all N in the muds is organic. Relationships between Corg and Porg and S are statistically insignificant. A significant positive relationship indicates a coupling of Pdet with Porg and Paut in the brackish-water muds in AS5-PC2 (Table 4; Fig. 5c). This coupling can also be seen in Fig. 4, where the vertical distributions of Pdet, Porg and Paut are markedly similar in the mud section of AS5-PC2. The strong coupling suggests that similar processes regulate temporal changes in the influx of detrital P and organic P, and the subsequent transformation of the latter to P-rich minerals, in the brackish-water muds at the AS5-PC2 site. A relationship between Pdet and PCDB is statistically insignificant, indicating decoupling of their supply. 5. Discussion 5.1. Waning glacial influence on deposition A thick water column immediately submerged the study area at the retreat of the Fennoscandian ice sheet. The ice margin retreated from the study sites soon after 11350 ± 110 cal. BP (Virtasalo et al., 2007). The water level was more than 100 m above the present level during the subsequent glaciolacustrine deposition of rhythmites (Glückert, 1995). Their rhythmic structure reflects the seasonal character of the icesheet melting; the coarser (silt–fine sand) couplet layers are deposited by underflows derived from the ice front, while the finer (clay) couplet layers are deposited by fall-out from the suspension (Sauramo, 1923). Deposition and geochemical component accumulation rates were high close to the ice front (Fig. 4). The rhythmites are poor in organic matter, which is a consequence of high input of glaciclastic material by subglacial melt river systems. Corg/Preac ratios (Table 3, Fig. 4) are markedly below the Redfield ratio of 106 (Redfield et al., 1963), indicating that a major part of the scarce organic matter is old and highly degraded organic matter. Fresh organic matter typically has high Corg/Porg ratios, while organic matter that has been extensively degraded prior to deposition has low Corg/Porg ratios (Ingall and Van Cappellen, 1990; Ruttenberg and Goñi, 1997). The absence of diatoms and trace fossils indicates very low production in the lake (Tuovinen et al., in press; Virtasalo et al., 2006, respectively). Thus, a major part of the organic matter in the rhythmites can be reworked from organic matter deposited before the ice covered the area. Redeposited pre-glacial organic-rich sediments have been documented from SW Finland (e.g. Donner and Gardemeister, 1971). Mean reactive Fe content in the rhythmites (0.16 wt.%) is similar to the average reported for glacial sediments (0.11±0.11 wt.%, Poulton and Raiswell, 2002). Reactive Fe predominantly is present as (oxyhydr)oxide nanoparticles in glacial sediments (Poulton and Raiswell, 2005). The presence of (oxyhydr)oxide nanoparticles suggests that not all mineral particles are pristine, which further supports that the rhythmites are a mixture of freshly crushed rock and reworked pre-glacial material. AlCDB, MnCDB and PCDB are strongly associated with reactive Fe (Table 4; Fig. 5a), which indicates little reductive dissolution in these deposits. Similar loadings of these Fe-(oxyhydr)oxide-associated components in PCA also suggest insignificant reductive dissolution (Table 5). It is concluded that the rhythmites were deposited under oxic conditions promoted by the high input of oxygen-rich meltwaters and by very low production in water. Clay varve studies indicate that the ice front was more than 300 km away from the study area at the onset of post-glacial lacustrine

9

deposition (Sauramo, 1923; Strömberg, 2005), which took place at 10 300 ± 100 cal. BP (Virtasalo et al., 2007). The lake level fell rapidly due to the glacio-isostatic land uplift; the water level was 85–45 m above the present level during the deposition of post-glacial lacustrine clays (Glückert, 1991). The first islands were emerging, and the shoreline advanced in the distance. Core lithology reflects the reduced glacial influence. The weakly-layered (non-varved) structure of the basal part of post-glacial lacustrine clays indicates that the underflows from the ice margin did not reach the study area any more; the poorly developed layering records fluctuations in the grain size distribution of the suspension, potentially due to seasonal differences in the icesheet melting (Virtasalo et al., 2007). The upward more homogeneous sediment structure reflects further reduction in the glacial influence, but the appearance of identifiable bioturbation structures (trace fossils) in the upper part of the succession also indicates intensified bioturbation and increased burrowing depths (Virtasalo et al., 2006). Also the dramatically lower delivery rates of geochemical components reflect the reduced glacial influence (Fig. 4). The post-glacial lacustrine clays are a mixture of sediments from distant sources such as meltwaters at the ice front, reworking of older deposits in shallow areas, and river load. The decoupling of Fe-(oxyhydr) oxide-associated components in the clays reflects the mixed origin. Also increased adsorption of Fe-(oxyhydr)oxide-associated components to clay particles may contribute to the decoupling. In addition, the strong opposite loadings of FeCDB and AlCDB to S and Paut on the PCA factor 1 (Table 6) suggest that the dissolution of Fe-(oxyhydr)oxides and precipitation of authigenic minerals such as sulphides control the behaviour of geochemical components in these deposits. Previous studies indicate post-depositional sulphidisation of the lacustrine clays due to the downward diffusion of excess H2S and sulphate from overlying brackish-water muds (Boesen and Postma, 1988; Sternbeck and Sohlenius, 1997; Böttcher and Lepland, 2000). Reactive Fe decreases and S increases in the top bluish-grey facies of the post-glacial lacustrine clays (Table 3), which can be a consequence of Fe-(oxyhydr)oxide dissolution due to the post-depositional sulphidisation. Therefore, postdepositional overprinting contributes to the poor coupling between the Fe-(oxyhydr)oxide-associated components, as well. However, the overprinting is restricted to the uppermost post-glacial lacustrine clays. It is rather well established that oxic seafloors dominated the Baltic Sea basin during the post-glacial lacustrine phase (Sternbeck and Sohlenius, 1997; Harff et al., 2001). Trace fossils in the upper part of the lacustrine clays indicate domicile-based activities (predation, scavenging and interface-dominated deposit feeding), which require aerobic conditions at the time of burrow emplacement (Virtasalo et al., 2006). Low organic contents in the post-glacial lacustrine clays are consistent with the oxic seafloor conditions (Fig. 3; Table 3). The organic contents are increased compared to the underlying rhythmites, which is in agreement with microfossil studies indicating higher primary production in the post-glacial lacustrine setting (Andrén et al., 2000; Tuovinen et al., in press). However, Corg/Preac ratios (Table 3) markedly below the Redfield ratio of 106 suggest also input of refractory organic matter (Ingall and Van Cappellen, 1990; Ruttenberg and Goñi, 1997). The admixing organics probably are pre-glacial organic matter transported from the ice margin, or reworked from older deposits as a result of the land uplift. Allochthonous contribution to the organic matter is further supported by the PCA factors 2 and 3, which suggest that the delivery of both primary organic matter and refractory organic P controls the behaviour of geochemical components in these deposits (Table 6). 5.2. Onset of brackish-water conditions The mid-Holocene glacio-eustatic ocean-level rise resulted in the transgression of the Atlantic into the Baltic Sea basin, and in the onset of brackish-water conditions in the Archipelago Sea soon after 7600 cal. BP (Virtasalo et al., 2006, 2007). Sedimentary microfossil studies indicate a marked increase in productivity in water at the onset of brackish-water

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conditions (Andrén et al., 2000; Tuovinen et al., in press). The increased productivity is recorded as a significant increase of organic and nutrient contents at the contact between the post-glacial lacustrine clays and the brackish-water muds (Fig. 3; Table 3). A similar organic content increase is recorded practically all over the Baltic Sea basin at the same level in sediments, i.e. at the so-called Ancylus/Littorina boundary (Winterhalter et al., 1981). The high organic deposition resulted in enhanced oxygen consumption and deteriorated seafloor oxygen conditions; the alternating thinly-laminated, weakly-laminated and strongly-bioturbated lithofacies of brackish-water muds record temporal fluctuations in the seafloor oxygenation (Morris et al., 1988; Virtasalo et al., 2005a, 2006). Detrital P contents decrease from AS5-PC2 to AS3-PC1 to AS2-PC4, reflecting increasing distance to a significant terrestrial source, the Paimionjoki river (Table 3; Fig. 1). This observation is in line the seaward decreasing Pdet contents on the modern seafloor of the Archipelago Sea (Virtasalo et al., 2005a). Also contents of reactive Fe and associated components decrease with distance to the Paimionjoki river. Reactive Fe and associated components (PCDB, MnCDB, AlCDB) are dominantly loaded on a different PCA factor than Pdet (Table 7), suggesting that reactive Fe and associated components are decoupled from the detrital flux. This decoupling probably results from the emergence of salinity gradients at the river mouths due to the transition to brackish-water conditions; reactive Fe (and associated components) is preferentially removed from the riverine particulate flux in estuaries by deposition into inner shore reservoirs and by the dilution of the flux by reactive Fe-depleted particles (Poulton and Raiswell, 2002). This decoupling is concordant with previous studies indicating that Fe(oxyhydr)oxides in the modern Baltic Sea are dominantly formed at sea (Bernard et al., 1989). Positive relationships between the reactive Fe and associated components (Table 4; Fig. 5b) suggest adsorption or coprecipitation either in the water column or in the surface sediments. Strong loadings of geochemical components associated with the redox-dependent dissolution Fe-(oxyhydr)oxides and P-rich apatite authigenesis on the PCA factor 1 (Table 7) suggest that redox-driven remineralization regulates the behaviour of geochemical components in the brackish-water muds. This interpretation is in line with earlier observations indicating that post-depositional processes of early diagenesis govern the elemental composition of the modern Baltic Sea sediments (Brügmann et al., 1992). Enrichment of the weakly-laminated facies in reactive Fe and associated components may be controlled by temporal changes in the seafloor redox state (Table 3). The weakly-laminated facies are taken to record time periods of irregular, supra-annual fluctuation in the seafloor oxygen conditions. Fluctuating redox conditions cause reoccurring dissolution of Fe-(oxyhydr)oxides, which results in the dominance of young Fe-(oxyhydr)oxides on the seafloor. Young Fe(oxyhydr)oxides are amorphous with high reactivity and a lot of sorbtion sites (Gunnars et al., 2002). The seafloor areas that suffer from irregular redox fluctuations in the modern Archipelago Sea are enriched in Fe(oxyhydr)oxides and associated components due to this process (Virtasalo et al., 2005a). The same process can also explain the lower contents of Fe(oxyhydr)oxides and associated components in the strongly-bioturbated facies indicative of predominantly aerobic seafloor conditions. Fe(oxyhydr)oxides transform into a more crystalline form with time, and their reactivity and adsorbtion capacity is reduced substantially (Lijklema, 1980). As a result, Fe-(oxyhydr)oxides in predominantly oxic seafloor areas in the modern Archipelago Sea have lower contents of Fe-(oxyhydr)oxides and associated components (Virtasalo et al., 2005a). The more reducing depositional history of the laminated facies compared to bioturbated facies is also shown by the regression analyses (Table 4; Fig. 5b). The y-intercept values of PCDB/FeCDB, MnCDB/FeCDB and AlCDB/FeCDB regression lines are more negative in the laminated facies of the brackish-water muds, which suggests that the share of PCDB, MnCDB and AlCDB not associated with Fe-(oxyhydr)oxides increases with the poorer redox state of sediments. This increase probably results from that a larger portion of reactive Fe is present as monosulphides in the reducing sediments due to reductive dissolution of Fe-(oxyhydr)oxides.

Authigenic P and S are enriched in the strongly-bioturbated facies (Table 3), which suggests enhanced authigenesis of P-rich apatite and sulphides in these deposits. The relatively high accumulation rates of organic-rich brackish-water muds results in the rapid burial of sediments into the reducing parts of sedimentary column (for general discussion, see Canfield, 1994), thus allowing post-depositional precipitation of sulphides also in bioturbated facies. PCA loadings of Porg and PCDB are opposite to Paut (Table 7), which suggests apatite authigenesis at the expense of organic and Fe-(oxyhydr)oxide bound P (Ruttenberg and Berner, 1993; Delaney, 1998). Recent studies on (pore water and) solid phases of P indicate that apatite authigenesis is an ongoing process in the recent Baltic Sea sediments (Carman and Jonsson, 1991; Jensen et al., 1995; Virtasalo et al., 2005a). Brackish-water muds in AS2-PC4 and AS3-PC1 have Corg/Porg ratios (Table 3) higher than the Redfield ratio of 106 (Redfield et al., 1963). The high Corg/Porg ratios are probably a consequence of the rapid organic matter burial into the reducing parts of the sedimentary column, where P is preferentially remineralized compared to C (Ingall and Van Cappellen, 1990; Delaney, 1998; Anderson et al., 2001). Similar PCA loadings of Corg and N (Table 7) suggest primarily marine origin for the organic matter. However, laminated mud facies in AS5-PC2 have Corg/ Preac ratios markedly below the Redfield ratio (Table 3), indicating admixing of previously degraded organic matter at that site (see below). 5.3. Direct climatic influence on deposition Redox histories of brackish-water mud sections in the studied cores differ markedly (Fig. 3). Different redox histories between the studied subbasins suggest that the considerable lateral variation of modern seafloor oxygen conditions, caused by the patchy topography, sluggish bottom water exchange and high organic deposition (Virtasalo et al., 2005a), has characterised the Archipelago Sea for a long time. Temporal changes in the seafloor oxygen conditions can result from shifts in the local current and sedimentation patterns due to the ongoing glacio-isostatic land uplift. Also human activity can have affected the seafloor oxygen conditions, but that is difficult to quantify based on the available data. Yet, it is intriguing that the changes in the sediment laminated structure more or less coincide with the known Holocene climatic phenomena. The lower laminated interval of brackish-water muds in AS5-PC2 was deposited during 1250–450 cal. BP (700–1500 AD) (Fig. 4). This time interval is more or less contemporaneous with the Medieval Warm Period recorded, for example, in two boreholes on the Greenland Ice Sheet (DahlJensen et al., 1998) and in lakes in Finland (Tiljander et al., 2003; HaltiaHovi et al., 2007). During this time interval, sedimentary microfossils and geochemistry indicate higher production and organic deposition (Andrén et al., 2000; Dippner and Voss, 2004), and laminated sediments imply anoxia (Harff et al., 2001), in the central Baltic Sea basin. The laminated interval in AS5-PC2 is enriched in all the P species (Fig. 4), indicating enhanced P delivery during deposition. Because Porg and Paut at the AS5 site are strongly coupled to Pdet (Table 4; Fig. 5c) that primarily originates from land, the increased P influx is taken to reflect increased terrestrial delivery of organic P, and its subsequent transformation to authigenic Prich minerals in the sediment. Low Corg/Preac ratios in this laminated interval further support temporal admixing of degraded terrestrial organic matter (Fig. 4). Recent climate models for the study area propose that a warmer climate will result in increased precipitation, and lead to higher river runoff and land erosion (Jylhä et al., 2004; HELCOM, 2007). Likewise, the medieval warmth may have enhanced precipitation, river runoff and land erosion, leading to elevated terrestrial P and organic supply. The AS5-PC2 site is located at the end of an elongated bay (Fig. 1), where the Paimionjoki river enhances the terrestrial influx. The higher P supply can have enhanced primary production in the water thereby fostering anoxia in a positive feedback, and leading to the laminated sediment deposition. The enrichment of PCDB in the laminated interval is probably a secondary effect because the Fe-(oxyhydr)oxide-associated components are decoupled from the detrital flux; the higher terrestrial P

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supply can have enhanced adsorption or co-precipitation of P in the water column or in the surface sediments. The cores AS2-PC4 and AS3-PC1 do not record anoxia during the medieval times. This can be due to that: 1) the brackish-water mud section in AS2-PC4 is short, indicating less accumulation and, subsequently, lower oxygen consumption and smaller probability for oxygen depletion; 2) the mud section in AS3-PC1 is of comparable length to that of AS5-PC2, but the AS3-PC1 site may not have been as severely affected by the temporal variations in seafloor oxygenation and terrestrial P flux because it lies at a 14 m higher elevation and is situated ~ 10 km farther away from the Paimionjoki river. Bioturbated sediments indicate oxic seafloor conditions at the AS5PC2 site during 450–300 cal. BP (1500–1650 AD) (Fig. 4). This coincides with the ‘Little Ice Age’ (Dahl-Jensen et al., 1998; Tiljander et al., 2003; Haltia-Hovi et al., 2007), when sedimentary microfossils indicate a colder climate for the Baltic Sea basin (Andrén et al., 2000; Emeis et al., 2003). The colder climate can have resulted in reduced production in water and improved seafloor oxygen conditions. The colder climate can also have resulted in decreased precipitation thereby reducing the terrestrial P influx and further suppressing productivity. The latest deterioration of seafloor oxygen conditions is dated to 300 cal. BP in AS5-PC2, and 200 cal. BP in AS3-PC1 when assuming a deposition rate similar to that in AS5-PC2 (Fig. 4). The onset of deterioration is contemporaneous with the temperature rise that followed the Little Ice Age (Dahl-Jensen et al., 1998; Tiljander et al., 2003; Haltia-Hovi et al., 2007). PCDB, Porg, Corg and N contents increase at this level in both AS5-PC2 and AS3-PC1, indicating increased organic accumulation (Fig. 4). The climate warming and increased organic deposition (primary production) probably enhanced seafloor anoxia. Because the laminated intervals in AS5-PC2 and AS3-PC1 extend to the modern seafloor, the positive feedback between the sea bottom oxygen deficiency, increased P regeneration and enhanced primary production can have resulted in prolonged oxygen depletion from which the both sites are suffering from even today. The apparent causal connections between the seafloor oxygenation and the Holocene climate variability suggest that direct climatic influences became dominant in the area after the glacio-eustatic ocean-level rise and the glacio-isostatic adjustment in the threshold area (Kattegat) to the Atlantic levelled out. For the past 4000 years, the Baltic Sea has been strongly affected by the North Atlantic climate that controls the freshwater runoff and irregular pulses of ocean water into the basin (Hänninen et al., 2000; Burke and Kemp, 2004). The link between the saltwater inflows and subsequent P regeneration in the modern Baltic Sea (Kahru et al., 2000) can have persisted for a long time.

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also resulted in higher primary production and organic (and nutrient) deposition, leading to fluctuating seafloor oxygen conditions. However, terrestrial organic influx can temporally overprint the marine organic deposition in areas close to rivers. Seafloor anoxia is a natural phenomenon in the brackish-water Archipelago Sea. Temporal changes in the seafloor oxygenation are broadly linked to the Holocene climate variability. On a finer scale, seafloor redox histories can be different in adjacent sub-basins due to patchy topography, sluggish bottom water exchange and high organic deposition in the area. These findings are important to keep in mind when assessing human influence on oxygen deficiency in the area. Acknowledgements The Finnish Graduate School in Geology supported this research. Funding for the geochemical analyses was received from the Finnish Cultural Foundation, K.H. Renlundin säätiö and Emil Aaltosen säätiö. Comments by Thomas Leipe (Baltic Sea Research Institute Warnemünde) and three anonymous reviewers helped to improve this manuscript. The crew and scientists onboard the R/V Aranda Cruise 12/2003 are thanked. Appendix A Age–depth relationships for calibrated AMS-14C age ranges in the core AS5-PC1 (Table 2) with the 95.4% probability area shaded. Dashed line indicates the linear sedimentation rate calculated based on the mean value of the age range determined at 276.5 cm.

6. Conclusions Observed variations in geochemical components for Holocene Archipelago Sea deposits reflect different depositional environments and post-depositional processes. These include the waning influence of the Fennoscandian ice sheet, and the dominance of direct climatic influence on deposition after the glacio-eustatic ocean-level rise and the glacioisostatic adjustment in the threshold area to the Atlantic levelled out. The studied geochemical components are strongly coupled in the basal glaciolacustrine rhythmites as a result of rapid deposition by glacial meltwater. Low Corg/Preac ratios suggest that a part of the rhythmite material is reworked from pre-glacial deposits. Geochemical component accumulation rates are drastically lower in the postglacial lacustrine clays due to an order-of-magnitude decrease in the sedimentation rate. Reactive Fe and associated components are decoupled in the post-glacial clays due to the increasing contribution of sediment from the emerging shoreline and rivers, but also to overprinting by post-depositional sulphidization. The onset of brackish-water conditions led to the establishment of salinity gradients at the river mouths, causing decoupling of reactive Fe from the detrital supply. The transition to brackish-water conditions

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