Features of chemical element distribution in pore waters and sapropel in the Dukhovoe Lake as reflection of the processes of freshwater diagenesis

Maltsev A.E., Leonova G.A., Bogush A.A., Bobrov V.A.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

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Chemical analysis data of pore waters possesses high information capacity being a sensitive indicator of very multifarious processes taking place in bottom sediments just as at the stage of evolution of the sedimentation basins, so during subsequent transformation of the diagenetic nature [1, 2]. We have investigated the chemical composition of pore water and sapropel of the Holocene sequence (7 m thick) from the Dukhovoe Lake located on the eastern side of the Baikal depression (53º 18' northern latitude, 108º 53' eastern longitude). The lake is shallow (~ 2.8 m in depth), it is 2500 m in length and 1600 m in width. Ecological conditions characterizing the current state of the lake (shallowness, the absence of water stratification with respect to oxygen and temperature) are favorable for mass generation of microscopic aggregations of phytoplankton (16 species of Chlorophyta, 9 species of Cyanophyta, and 8 species of Bacillariophyta) that is the main producer of sapropel organic matter. Values of C_org in phytoplankton (21 %) close to those in the upper (0–2 cm in thickness) sapropel horizon (22 %) testify indirectly to the fact that all organic matter of the phytoplankton is “succeeded” by bottom sediment [3].

A continuous capropel core (7.5 cm in diameter and ~7000 cm in length) was taken from the Dukhovoe Lake in July 2008 with the help of percussion drilling. Pore water was squeezed from core layers 10 cm thick applying the standard procedure [4]. In the same layers, the microelement composition, the total content of organic matter as well as the ash factor that is opposite to the later, and C_org were determined. Fig. A shows the irregular distribution of C_org throughout the core depth. Concentrations of C_org are 18.5–25.7 % within the depth vary in range 1–180 cm; below this interval, sharp decrease up to 10% is observed, and then a smooth transition to values 5–2% at the depth of 550 cm takes place. The distribution of the ash content is diametrically opposite to that of C_org, which increases downward the section from 32.2% to 87.2 %. Analysis of the microelement composition for the entire sapropel section allows one to judge about specific geochemical character of the sapropel such as the low content of Ca and Fe and the sufficiently high content of U. This is conditioned by a nature of phytoplankton as material forming sapropel. Phytoplankton, as a rule, does not accumulate Ca and Fe abundantly as opposite to macrophytes and macrophytegenic sapropel [5]. Such relatively high U contents just as in sapropel itself (9.7–22.2 mg/kg dry material), so in phytoplankton (2.5 mg/kg) is most probably explained by general geological feature of the Baikal and Transbaikal regions, where numerous small shows of uranium and uranium- and thorium-bearing mineralization (placer monozite deposits) are revealed.

According to Alekin’s classification [6], we assign pore water from sapropel of the Dukhovoe Lake to the sulfate-calcium water type (SO₄–Ca), and to waters with increased mineralization (320–3514 mg/L) with respect to the content of dissolved salts. [7]. The pH values in the water vary from 3.9 to 9/6 in the core depth (Fig. B). In pore water of the upper 10 cm sapropel layer, minimum pH values (3.91) are observed at the depth of 232 cm and remain to be within the 5.02–5.19 to the depth of 320 cm, and then pH sharply increases to 9.4 at the depth of 340–360 cm. In the lower core part, рН values of pore water become neutral, and then these gradually increase over the core section up to the depth of 160 cm (рН = 6.44–7.09), and newly decrease in pH values to lower layers is traced. Minimum pH values such as 3.91 are observed at the depth of 232 cm in the range 5.02–5.19 to the depth of 320 cm, whereupon the very sharp increase in рН to 9.4 is observed at the depth of 340–360 cm. In the lower core part, рН values of pore water are close to neutral. The reduction oxidation potential (Eh) of the pore water varies in the range from +167 to +425 mV, the pattern of the distribution of Eh throughout the core is opposite to that of рН (Fig. B).

Given in Fig. С are most specific concentration profiles for distribution of (SO₄^2– and Ca²⁺), Fe and Mn ions in pore water in the core depth. These ions are sensitive indicators of variation in physic-chemical conditions in sediments, as well as biophile elements Zn and Cu, and industrial elements such as Pb and Cd characterizing pollution of the atmosphere. Variation in the concentrations of SO₄^2– and Ca²⁺ ions in pore water in depth occurs nearly synchronously. With increase in the concentration of SO₄^2–, the Ca²⁺ concentration also increases, and vice versa.

When distributions of Mn and Fe concentrations and Eh values in pore waters in the core depth are compared (Fig. С), the following preliminary conclusion can be drawn. For Eh values (~ 300 mV) being kept to the core depth of 160 cm, invariable Mn concentrations are also observed..When Eh values sharply increase to 400 mV within the depth interval 160–320 cm, the Mn content also sharply increases in pore water. Downward the core section, the Mn content decreases following the decrease in the Eh values. On the contrary, for Fe, with sharp increase in Eh values, the Fe content in pore water sharply decreases at the expense of its involvement into formation of solid mineral phases.

The distribution of Cu and Zn in pore waters reflects, as a whole, variation of pH values in these waters. The maximum Cu content (0.12 mg/L) corresponds to the least values of рН = 3.91 at the depth of 262 cm. With sharp increase in рН values to 7.56, the Cu content becomes a half as much as its maximum content (0.064 mg/L). This is explained by greater mobility of many elements in an acid medium as compared to an alkaline medium. The highest Zn concentrations in pore waters are associated with horizons of organic matter (1–200 cm) alternating with beds of clay material. Higher Zn concentrations are observed in porous water with low pH values as this is for Cu. Concentrations of Pb and Cd in pore water are sufficiently invariable throughout the core depth except upper layers with increased contents of these elements. Such distribution of “volatile” elements in bottom sediments in small lakes and it reflects atmospheric pollution from industrial pollution.

Fig. Distribution of С_org and the ash content throughout the core from the Dukhovoe Lake (A), pH and Eh values (В), and chemical elements (С) in pore water

As a whole, it is clear that the sharp increase in concentrations of all studied elements, except Pb, in pore waters occurs within the core interval of 160–320 cm. Consequently, the intense leaching-out of elements takes place in this interval. This is entirely consistent with the рН distribution. Within these horizons, the sharp pH decrease up to its acid values is observed that stipulates leaching-out of elements from sapropel material. Below the depth of 320 cm, the sharp decrease in concentrations of elements in pore waters and increase in pH values take place. This may stipulate formation of a geochemical barrier and concentration of elements at this depth.

This research was supported by the RFBR (grant 08-05-00392, 11-05-00655 and №11-05-12038-ofi-m-2011)

References:

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