Chapter 4 Mountain mires from Galicia (NW Spain)

Peatlands: Evolution and Records of Environmental and Climate Changes I.P. Martini, A. Martı´ nez Cortizas, W. Chesworth, Editors r 2006 Elsevier B.V. All rights reserved.

85

Chapter 4

Mountain mires from Galicia (NW Spain) X. Pontevedra-Pombal, J.C. No´voa Mun˜oz, E. Garcı´ a-Rodeja and A. Martı´ nez Cortizas

Introduction Galicia, on northwestern Iberian Peninsula (411 480 –431 480 N latitude, 61 420 –91 180 W longitude), is a coastal area in a transition zone between temperate and subtropical climates, and where mires are relatively abundant (Fig. 4.1). The most recent estimates indicate that Galicia has some 10,000 ha of mountain mires, which represents a 0.4% of the territory (Pontevedra-Pombal and Martı´ nez Cortizas, 2001; Pontevedra-Pombal, 2002). The Galician mires have been studied since the first decades of the 20th century, mostly for palynology (Casares Gil, 1920; Jato, 1974; Ramil, 1992; Mun˜oz Sobrino, 1996). Recently, research has been focused on a more general approach to understand the ecosystems. In the last 10 years, a large number of mires were systematically characterized for their physicochemical properties (Pontevedra-Pombal et al., 1996, 2001b), organic chemistry and dynamics of the peat composition (Pontevedra-Pombal et al., 2001a, in press), understanding the whole ecosystem development (Martı´ nez Cortizas et al., 2001; Pontevedra-Pombal, 2002), and interpreting the environmental signals preserved in the peat, which provide information on the composition of the atmosphere as well as about climatic and human interference during the Holocene (Martı´ nez Cortizas et al., 1999, 2002, 2004). Mapping of the mires has also constituted an essential tool for the evaluation of their scientific potential, as well as a basic element for decision making related to protection issues (Pontevedra-Pombal, 2002; Pontevedra-Pombal et al., 2003).

Mires development Distribution and biogeographical conditions The location of the principal mountain mires of Galicia (NW Spain) is shown in Figure 4.2. These mires occur in three major areas: northern, coastal and pre-coastal ISSN: 0928-2025

DOI: 10.1016/S0928-2025(06)09004-3

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Figure 4.1. Simplified map of Galicia, NW Spain.

Figure 4.2. Galician mountains: main areas and peatland coverage (gray areas).

Mountain mires from Galicia (NW Spain) Table 4.1. Galicia.

87

Environmental characteristics of the three main spatial units of the mires of

Environmental parameters

Peat mechanism

Coastal mountains and Dorsal Galega

Eastern mountains

Northern mountains

High precipitation Moderate average temperature Abundant fluvial network Moderate freeze–thaw processes Moderate oceanity

High precipitation Low average temperature Abundant fluvial network Strong glacial and periglacial paleoprocesses Continentality

High precipitation Low average temperature Very abundant fluvial network Intensive glacial and periglacial paleoprocesses Strong oceanity

Paludification

Terrestrialization

Paludification; Terrestrialization

Table 4.2. Climatic conditions of the mires main areas of Galicia (data from Martı´ nez Cortizas and Pe´rez Alberti, 2000). Area

Precipitation

Temperature

Alt

Win

Spri

Sum

Aut

An

Win

Spri

Sum

Aut

An

North

600 800 1000

492 561 635

275 314 355

150 172 194

477 545 616

1394 1592 1800

6.3 5.1 3.9

9.9 8.4 6.9

14.6 13.2 11.7

9.1 7.9 6.7

10.0 8.7 7.4

East

1000 1200 1400

480 531 582

291 322 353

139 154 168

490 594 697

1400 1600 1695

4.2 2.3 0.3

10.3 8.2 6.1

16.0 14.2 12.4

6.8 4.9 3.1

9.4 7.4 5.5

Southeast

1000 1400 1800

428 536 644

256 320 384

120 151 181

438 547 657

1242 1554 1866

4.2 1.9
0.4

10.6 8.1 5.6

16.8 14.7 12.6

6.9 5.3 3.7

9.6 7.5 5.3

Notes: Alt, altitude; Win, winter; Spr, Spring; Sum, summer; Aut, autumn; An, annual. Precipitation in mm; temperature in 1C.

(mountains that separate the littoral area to the continental area), and eastern areas. Environmental and paleoenvironmental conditions in these three areas have conditioned the kind and intensity of processes associated to the formation of mires (Table 4.1). In peatland areas, annual precipitation ranges from 1200 mm in the southeastern areas (at 1000 m asl) to 1800–1900 mm in the most elevated ranges of the northern and eastern areas (Table 4.2). Mean annual temperatures vary between 5.3 and

X. Pontevedra-Pombal et al.

88

Table 4.3. Rainfall and temperature mean gradients and constants in the north and east of Galicia (data from Martı´ nez Cortizas and Pe´rez Alberti, 2000). Area

Temperature (oC)

Rainfall (mm)

Gradient (mm/100 m altitude) Constant Gradient (mm/100 m altitude) Constant North 100 East 75

795 647


0.67
0.50

14.1 14.2

10.3 1C. The oceanic character of the region is affected by the winter–summer equilibrium between temperate and subtropical components; the later is responsible for the rainfall seasonality (Table 4.3), which is more pronounced to the south (Martı´ nez Cortizas and Pe´rez Alberti, 2000). Present distribution of mires is the result of the interaction between several factors at different spatial and temporal scales, and their formation must be understood in paleoenvironmental terms. Upper Pleistocene–Holocene climate changes, a complex relief in terms of forms and spatial structure, and the predominance of acid rocks have generated diverse ecological conditions that favored the development of this kind of wetlands ecosystems. Geomorphologic criteria are essential to understand the development of Galician mires. Their location and typology is to a great extent linked to relief forms, generated under glacial and periglacial climatic conditions during the last phases of the Wu¨rm glaciation. In the northern coastal areas cold-climate processes regularized mountain summits, creating flat surfaces where paludification processes began as soon as the climate improved. In this area, due to the great quantity of rain, mires developed quickly and extended in all directions forming blanket mire macrotopes. On the other hand, in the old glaciated areas of the eastern and southeastern areas, erosive as well as sedimentary forms generated small basins (ponds) where terrestrialization prevailed in the development of mires. In terms of vegetation, Fraga Vila et al. (2001) elaborated a list of 182 species present in NW Spain mountain mires. Of these, 133 are vascular plants, 46 are bryophytes (11 of the genus Sphagnum) and 3 lichens. The greatest plant diversity was found in minerotrophic mires, whereas ombrotrophic mires showed a much lower biodiversity. Species with large ecological amplitudes, like Molinia caerulea and Festuca rubra, are represented in all types of mires, although with different abundance. On the contrary, some plants are specific of one area, as for example Carex duriaeui and Erica mackaiana, which are characteristic of the northern area, or Carex nigra and Erica tetralix in the eastern area. Typology Using a hierarchic classification system of mires (Fig. 4.3), three main types of mires occur in these mountainous areas: blanket bogs (ombrogenic mires), fens (minerogenic mires) and raised bogs. These are comparable to those established for other Atlantic areas in Europe (Moore and Bellamy, 1974; Davis and Anderson, 1991; Lindsay, 1995).

Mountain mires from Galicia (NW Spain)

89

Figure 4.3. Hierarchic classification system of Galicia mountain mires.

Ombrogenic mires are limited to the northern area, where they occur directly over acid rocks, periglacial stony deposits, weakly evolved soils or rarely over podzolic soils. Fens are well represented in all mountainous areas. The oldest fens are located in the eastern areas on small glacially excavated areas, and some date back to 17,000 14 C yr BP. In the northern area, fens occupy large depressions, up to a meter deep and 55 ha wide, formed by weathering and erosion of granites. Locally some fens have evolved into raised bogs. Owing to their high environmental value, the European Union (EU) included mire ecosystems in the Habitats directive and in the Natura 2000 framework, as ecosystems of preferential conservation (European Commission, 1996). Peatlands from Galicia can be included in the group RAISED BOGS AND MIRES AND FENS, Sphagnum acid bogs, and specifically in the habitats defined as: 7110-Active raised bogs; 7120-Degraded raised bogs still capable of natural regeneration; 7130-Blanket bog, active only; and 7140-Transition mires and quaking bogs.

90

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Figure 4.4. Location of blanket bogs in Europe (black areas; after Raeymaekers, 1999).

Figure 4.5. Active blanket mire complex of Barreiras do Lago (Serras Septentrionais, Galicia).

Raeymaekers (1996) limited the distribution of blanket mires in Europe to western Ireland, Scotland, central Sweden and French Bretagne (Fig. 4.4). Our work has revealed the presence of this type of mire in Galicia as well (Figs. 4.4, 4.5; Pontevedra-Pombal, 2002). Furthermore, Raeymaekers (1996) reported the presence of raised bogs from the British Isles, Sweden, Finland, from elevated mountains of France, Switzerland and Italy, and in The Netherlands (Fig. 4.6). This type also

Mountain mires from Galicia (NW Spain)

91

Figure 4.6. Location of raised bogs in Europe (black areas; after Raeymaekers, 1999).

occurs in Galicia, although it is not the most abundant peatland, and shows a characteristic dome at the center.The last type, transition mires and quaking bogs, is the most abundant. We have identified both subtypes: those developed on slopes (transition mires) and those developed on depressions (quaking bogs) such as glacial or fluvial valleys, weathering basins, over-excavated areas and depressions between moraines. From a geochemical and hydrological point of view, the first subtype represents a gradation between minerogenic and ombrogenic conditions. The second subtype is usually hydrologically connected to the slope areas, so it grades into transition mires or aerobic wet soils rich in organic matter. This spatial connection is responsible for both the morphology and stratigraphy of these mires.

Chronology Data from 30 representative mires indicate that peat accumulation in fens of the eastern and southeastern areas began at least 11,000–10,000 14C yr BP by terrestrialization (Fig. 4.7). These are the areas most intensely glaciated during the latest Pleistocene (Valca´rcel, 1999). Radiocarbon dates of 17,300–17,400 14C yr BP of the basal peats at Lagoa de Lucenza–Serra do Caurel indicate a rapid onset of peat formation after the Last Glacial Maximum (20,000–18,000 14C yr BP). However, the most intensive fen formation occurred during two main episodes, one around 5000–4000 14C yr BP, and the other between 3000 and 2000 14C yr BP. The formation of blanket mires started 9000–8000 14C yr BP, and an intense period of bog formation occurred between 6000 and 5000 14C yr BP.

92

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Figure 4.7. Initial phases of peat formation in peatlands (mires) of different areas of Galicia (age in 14C yr BP).

The formation and expansion of Galician mires is related to climatic and soil evolution as well to human activities since prehistoric times. Many authors have indicated the importance of global climatic oscillations on the development of mires in Europe (Ratcliffe, 1977; Solem, 1986; Averdieck et al., 1993). Malmer (1975) reported that a vast region of United Kingdom is constituted by blanket bogs developed by paludification after the sub-Boreal period, some 3000–2500 14C yr BP, probably as a consequence of the climate degradation during the sub-Boreal/sub-Atlantic transition, which resulted in an elevation of the water table in most mineral soils of western Europe. On a larger scale, this degradation also resulted in a generalized paludification of the highlands with the development of iron pans and the accumulation of organic matter. In pedogenetic terms, blanket mires are considered as the final stage of soil maturation in areas with high precipitation, thus their formation is essentially pedogenetic, and associated with the intense leaching of redox-dependent elements. In Great Britain, blanket mires are related to podzolization processes responsible for the formation of iron crusts and impeded water movement (Godwin, 1946). Although Dimbleby (1985) indicated that mires rarely develop over podzolic soils, some Galician mires have an accumulation of iron oxy-hydroxides at their base, at the contact with the sediment or rock. The studies about polycyclic soils near peatlands area have identified important extensions of accumulation of iron oxy-hydroxides layers. In some areas, these layers have generated an intense paludification and stimulated the formation of peatlands. Pollen records obtained in Galician mires show a forest regression phase, coincident with the accumulation of inorganic layers in some fens (To¨rnqvist and

Mountain mires from Galicia (NW Spain)

93

Joosten, 1988; Ramil, 1992). Some studies on soil erosion, archeological records and landscape evolution during the Holocene also provide good evidence that these erosion episodes are coeval with the development of different human cultures in the area (Epipaleolithic, Neolithic, Bronze Age, Iron Age) (Benito et al., 1991; Martı´ nez Cortizas and Moares, 1995). So it is possible that human-induced erosion may have promoted increased water runoff in slope areas and elevated water tables in the lowlands, as also proposed in other areas (Moore, 1975; Chambers, 1988). The high charcoal content in the basal soils of some mire supports this interpretation. Growth and accumulation rates Average maximum depth of minerotrophic mires in Galicia is 2–3 m, although in the eastern and southeastern areas it may reach up to 7 m, but in the latter not all layers are organic. Ombrotrophic mires have depths of up to more than 5 m, with 3 m being a representative depth. This depth is greater than the 2.0–2.5 m reported for boreal and subarctic European and North America mires (Riley, 1987; Gorham, 1991). Age–depth relationships are shown in Figure 4.8. Most ages of Galician mires fit to a linear function with a slope of 21 (n ¼ 55, correlation coefficient r ¼ 0.98). This

Figure 4.8. Peat age–depth relationship of Galician mires. The points represent age of basal peats from different mires, and the black and gray lines indicate linear functions through two groupings of mires (age in 14C yr BP).

94

X. Pontevedra-Pombal et al.

indicates that average peat accumulation during the Holocene was 1 cm every 20–25 years. Some radiocarbon ages of different mires also fit to a linear function with a similar slope (b ¼ 23; n ¼ 15, r ¼ 0.88), but shifted some 4000 14C yr BP indicating the presence of a discontinuity or a change in peat accumulation. The almost identical slope indicates that peat accumulation rates were the same for all mires, but the shift (different ages for the same depth) suggests the second set of mires were affected by erosion processes or strong change of decomposition of the organic matter, which eliminated younger peat sections, or there was a stop or dramatic reduction in peat accumulation at some time. The latter is unlikely because a gap of 4000 years seems too large to be explained by changes in peat accumulation. Thus, this may correspond to a global episode since it is observed in mires from different areas, and, in many cases, coincides with erosive features, decay change of the peat and strong peat compaction. It also is similar to the Grenzhorizont or Boundary Horizon reported by different authors, a highly decomposed black peat layer below very low decomposed brown peat (Weber, 1908; Godwin, 1981). This boundary horizon has been identified in many areas in continental Europe and in UK, and it was interpreted as a reaction of mire ecosystems to a global climate change. Granlund (1932) and Ma¨kila¨ (1997) reported at least five such events/horizons with ages of 4300, 3200, 1600 and 800 14C yr BP. Peat growth rates in Galician mires vary between 0.2 and 0.7 mm yr
1, with average values of 0.45–0.47 mm yr
1 (Fig. 4.9). These rates are comparable to those reported for Europe and North America. Gorham (1991) suggested a value of 0.5 mm yr
1 as a reasonable conservative estimation for mires at a global scale, whereas the average rate for mires from northern Europe is 0.60–0.75 mm yr
1 (Aaby, 1986). Growth rates in NW Spain seem to be mostly related to time periods and mire type. Numerous investigations in other parts of the world too have demonstrated that growth rates are as variable as typology, latitudinal location and microhabitats

Figure 4.9. Average accumulation rates of peat of studied Galician mires in different areas.

Mountain mires from Galicia (NW Spain)

95

of mires. Rates vary between 0.84 mm yr
1 for the Florida Everglades (McDowell et al., 1969) and 0.025 mm yr
1 for certain Canadian ombrotrophic bogs (Boville et al., 1983). For concentric raised bogs from SE Finland, Ma¨kila¨ (1997) obtained rates of 0.76 mm yr
1 for Acutifolia, 0.67 mm yr
1 for Sphagnum, 0.56 mm yr
1 for Carex and 0.32 mm yr
1 for Scheuchzeria–Cuspidata communities. Silvola (1986) reported a range of 0.5–1.5 mm yr
1 for subarctic and boreal mires. The highest growth rates (0.6–0.7 mm yr
1) occur in mires that began formation 3000–2000 years ago, coinciding with the generalized growth increase indicated by Aaby and Tauber (1974) for northern Europe since 3000 14C yr BP, with a maximum in the last 1000 years. The growth models indicate that ombrogenic mires whose formation began at the onset on the Holocene show a higher growth rate than the minerogenic mires. When multiple radiocarbon dates from each profile of various mires are available, the calculated growth rates for ombrogenic mires are almost constant or show small variations. In Galicia, like other oceanic mires of North Hemisphere, this may be related to the relative greater effective precipitation in the oceanic areas where they are located. Mires can be considered sinks of a large number of chemical elements, and particularly carbon. Recent estimations of the total carbon reservoir in mires provided an average accumulation of 90–96  106 t yr
1 (Gorham, 1991), although Silvola (1986) has suggested a somewhat greater value (110  106 t yr
1). Carbon accumulation can be modeled by a non-linear function, if multiple ages are available for one mire. In this way Clymo (1991) calculated an average rate of C accumulation of 23 g m
2 yr
1 for 38 boreal mires. But, stratigraphic analyses of mires from different geographic areas have shown that accumulation rates have varied during their development, and that these variations correlate with local and global changes (Aaby and Tauber, 1974; Svensson, 1988). The variation in accumulation rates makes it difficult to assign overall average values for all mires. The basal age, the stratigraphy, C content and peat density have to be known for a proper estimation of the longterm rate of carbon accumulation (LORCA; Tolonen et al., 1992). This complete set of data is available only for some 200 mires from Europe and North America (Gorham, 1991; Zoltai, 1991; Tolonen et al., 1992; Korhola et al., 1995). In NW Iberian Peninsula, we have studied 10 mires in this way, and the resulting accumulation rates (Table 4.4) vary between 48 and 112 g m
2 yr
1 of dry mass (82.0724.2 g m
2 yr
1), and 18.7–48.9 g C m
2 yr
1 (31.1711.0 g C m
2 yr
1). When this information is Table 4.4. mires. Mire type

All Ombrotr. Minerotr.

Statistics of mass and carbon accumulation rates in different types of Galician Carbon accumulation rates (g m
2 yr
1)

Mass accumulation rates (g m
2 yr
1) Aver.

SD

Min.

Max.

Aver.

SD

Min.

Max.

95.1 82.0 95.8

33.9 21.0 32.0

48.1 48.1 53.5

168.9 108.6 146.5

37.5 39.8 29.3

14.6 9.0 9.8

18.7 25.0 18.7

72.6 48.9 41.4

Note: SD, standard deviation.

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X. Pontevedra-Pombal et al.

combined with that available in the literature for other mires of Galicia (Molinero et al., 1984; Aira and Guitia´n, 1986a, b; Ramil et al., 1994), we obtain, for the Holocene period, an average dry mass accumulation of 95.1733.9 g m
2 yr
1 and an average C accumulation of 37.5714.6 g C m
2 yr
1. Minerogenic mires have a higher mass accumulation (96 g m
2 yr
1) than ombrogenic mires (82 g m
2 yr
1), but the latter accumulated more carbon (39.8 versus 29.3 g C m
2 yr
1). This fact is related to the effect of the greater degree of decomposition and mineral matter incorporation in the density measurements in minerogenic mires. The average C accumulation rate in Galician mires is greater (37.5714.6 g C m
2 yr
1) than the global average value (20 g C m
2 yr
1) for mires given by Armentano and Menges (1986), as well as that reported for boreal Canadian mires (20 g C m
2 yr
1, Armentano and Menges, 1986; 29 g C m
2 yr
1, Gorham, 1991), Finland (o19 g C m
2 yr
1, Tolonen and Vasander, 1992; 25 g C m
2 yr
1, Silvola, 1986), Russia (20 g C m
2 yr
1, Armentano and Menges, 1986) and USA (19.9 g C m
2 yr
1, Korhola et al., 1995). But it is slightly lower to rates reported for European mires (48 g C m
2 yr
1, Armentano and Menges, 1986), W-NE USA (48 g C m
2 yr
1, Armentano and Menges, 1986) and the British Isles (450 g C m
2 yr
1, Ovenden, 1990). The C average accumulation rates of Galician ombrogenic mires (39.8 g C m
2 yr
1; range 25–49 g C m
2 yr
1) is higher than those average of ombrogenic mires from Scandinavia (9–34 g C m
2 yr
1; 20–30 g C m
2 yr
1, Tolonen, 1979), Estonia (22.5 g C m
2 yr
1) and USA (20–26 g C m
2 yr
1), but similar to those of raised bogs from Siberia and Finland (20–40 g C m
2 yr
1, Botch and Masing, 1983; 13–41 g C m
2 yr
1, Tolonen et al., 1992) or fall within the wide range reported for boreal mires (6.6–85.5 g C m
2 yr
1, Korhola et al., 1995). With regard to boreal minerogenic mires, Korhola et al. (1995) indicated C accumulation rates of 4.6–46.1 g C m
2 yr
1, which includes the range observed in Galicia (18.7–41.4 g C m
2 yr
1). But the average value for the later (29.3 g C m
2 yr
1) is greater than that of minerogenic mires from Finland (15.1 g C m
2 yr
1, Tolonen et al., 1994) and slightly higher than those of USA boreal minerogenic mires (27 g C m
2 yr
1, Tolonen et al., 1992) and France (28 g C m
2 yr
1, Francez and Vasander, 1995). Despite the high variability of C accumulation rates, coherent with differences in latitudinal distribution, mire type and local environmental factors, the results indicate that there is a qualitative global pattern in C accumulation between ombrogenic and minerogenic mires. So the greater C accumulation in ombrogenic as compared to minerogenic mires in Galicia is consistent with the process of formation of both types of mires in other distant geographic regions, as already pointed out by Korhola et al. (1995). In a research of more than 1300 mires from Finland, Estonia and USA, they observed that ombrogenic mires have accumulated more C (n ¼ 548, average ¼ 22.5 g C m
2 yr
1) than minerogenic ones (n ¼ 373, average ¼ 15.1 g C m
2 yr
1).

Composition and properties Most mires have superficial layers with low or very low decomposed fibric material, no deeper than 25 cm (Table 4.5). Below hemic peat extends for almost the whole

Physico-chemical range values of the different areas of peatlands on the northwest Iberian Peninsula.

Mire

Area

Organic horizon

Depth (cm)

Bulk density

Particle density

Porosity (%)

Ash (%)

PI

C

N

S

AGN˜1 BAG BDX1 BLA BMC1 BPA BRN2 BUI3 CAD CDL4 CPD LUZ5 MII6 MIM6 PDC PNV4 PVO PZC QXI7 SUA

N E SE E C E C N N N E E SE SE N N N E SE E

Oi-Oe-Oa Oi-Oa Oi-Oe Oi-Oe-Oa Oi-Oa Oi-Oe Oi-Oa Oe Oi-Oe-Oa Oa Oi-Oa Oi-Oe Oi Oi-Oe Oi Oi-Oe-Oa Oi-Oe Oi-Oe-Oa Oi Oi-Oe-Oa

130 135 40 145 70 190 110 400 124 400 55 540 140 100 184 250 300 265 105 115

0.04–0.20 0.06–0.60 – 0.11–0.26 – 0.1–0.34 – 0.11–0.19 0.13–0.98 0.15–0.23 0.14–0.17 – – – 0.09–0.16 0.07–0.32 0.09–0.24 0.16–0.60 – 0.10–0.23

– 1.40–2.20 – 1.53–1.88 – 1.50–1.86 – 1.90–2.20 1.34–2.06 – 1.54–1.66 – – – 1.36–1.51 – 1.39–1.53 1.72–2.22 – 1.45–1.83

– 55–85 – 86–93 – 81–93 – 87–92 60–91 – 89–90 – – – 88–94 – 83–94 74–91 – 77–93

– 4–63 – 10–25 – 21–57 – 4–19 2.7–79 – 19–34 – – – 1.3–7.3 – 1–12 54–76 – 5–50

2–5 0–7 – 2–7 – 3–5 – – 1–6 – 1–5 – – – 4–7 – 4–5 2–7 – 2–5

21–43 29–49 14–27 29–44 30–32 23–46 15–32 48–60 17–55 39–60 31–37 14–35 38–40 26–34 47–57 35–51 44–57 14–25 22–45 24–50

– 0.2–2.3 0.1–1.0 1.8–2.4 0.5–0.7 1.1–1.7 0.7–2.0 – 0.5–2.0 – 2.0–2.4 0.8–1.8 1.8 1.6–2.1 1.5–2.2 – 1.2–2.3 0.3–1.6 0.7–2.0 1.1–2.7

– 0.20–2.60 – 0.73–0.98 – 0.50–1.10 – – 0.06–0.30 – 0.79–0.97 – – – 0.19–0.39 – 0.61–0.77 0.30–1.10 – 0.54–0.95

pH H2O

pH KCl

PH CaCl2

BAG BDX BLA BMC BPA BRN

3.5–4.3 4.3–4.6 5.9–6.0 3.9–4.0 3.5–4.3 4.2–4.7

2.3–4.3 3.4–3.8 5.1–5.3 3.5–3.9 – 3.8–4.2

2.1–4.1 – 4.7–4.8 – – –

Ca 0.8–17.8 0.40–0.70 1.80–5.91 5.60–7.50 1.0–7.1 0.20–0.50

Mg

Na

K

Al

CEC

0.2–12.1 0.08–0.20 0.31–1.00 1.90–2.20 0.2–0.9 0.10–0.50

0.03–0.9 0.01–0.08 0.73–2.60 0.40–0.70 0.02–0.9 0.10–1.70

0.01–1.3 0.06–0.10 o0.01–2.32 o0.01–0.06 0.02–1.0 0.05–0.20

2.9–155 8.00–10.80 o0.01–0.09 7.10–15.90 2.5–13.4 5.00–10.00

11.3–221.3 44.7–78.8* 3.79–12.31 99.5–126.9* 8.3–16.2 60.0–165.5*

97

Mire

Mountain mires from Galicia (NW Spain)

Table 4.5.

98

Table 4.5 (continued ) BUI CAD CPD LUZ MII MIM PDC PVO PZC QXI SUA

3.2–3.8 2.5–3.9 4.3–4.7 3.4–4.8 4.6–4.9 4.7–4.9 3.6–4.4 3.6–4.6 3.4–4.8 3.8–4.2 4.5–5.6

– 2.5–3.0 3.9–4.3 3.1–3.8 4.4–4.6 4.4–4.6 2.9–3.3 2.3–3.3 3.4–4.2 2.8–3.6 3.6–4.2

– 2.0–2.4 3.2–3.7 – – – 2.9–3.2 2.3–3.1 3.3–4.1 – 3.0–3.9

o0.01 0.1–8.1 0.60–3.59 3.00–4.30 – 0.40–0.60 0.2–4.2 0.08–6.69 0.6–11.0 0.50–0.70 1.91–10.02

1.40–3.00 0.5–9.3 0.18–2.47 0.30–1.20 – 0.10–0.30 4.7–13.2 4.33–9.36 0.9–2.6 0.40–1.20 0.28–1.29

1.00–2.40 0.03–1.98 0.14–0.97 0.20–0.40 – 0.30–0.70 0.5–1.2 0.46–1.39 0.08–1.5 0.30–0.40 1.49–3.37

o0.01 o0.01–1.1 0.25–0.71 0.03–0.20 – 0.05–0.10 o0.01–0.5 0.02–2.93 0.09–4.2 0.07–0.30 o0.01–4.02

– 2.7–14 6.77–10.38 3.10–4.70 – 0.90–7.10 1.0–3.4 0.31–3.76 0.5–5.1 3.20–5.3 0.61–3.99

74.9–95.5* 7.1–23.5 8.77–17.35 42.2–56.0* – 23.8–30.7* 9.8–21.1 6.98–20.1 5.6–19.4 12.0–16.0 8.18–20.56

X. Pontevedra-Pombal et al.

Note: Area: N, North; E, East; SE, Southeast; S, South; C, Center. Horizon: present organic horizons; bulk and particle density in Mg m
3; ash, total carbon (C), nitrogen (N) and sulfur (S) in percentage; PI, pyrophosphate index; cations and CEC, cationic exchange complex in cmolc kg
1, where the values with asterisk have analyzed the CEC in ammonic acetate pH 7, and the rest have analyzed the CEC in ammonic clorure to field pH. The superscript numbers in the mire column reference to the source of the information: 1 Ramil and Aira Rodriguez (1993). 2 Leiro´s and Gutia´n Ojea (1983). 3 Molinero et al. (1984). 4 Ramil et al. (1994). 5 Aira and Guitia´n (1986a, b). 6 Torras (1982). 7 Sanmamed (1979).

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99

Figure 4.10. Bulk (BD) and particle (PD) density differences between an ombrogenic and a minerogenic mire. The profiles indicate the horizon types according to soil taxonomy (Soil Survey Staff, 1998).

depth of the mire. Only a few minerogenic mires have a more or less developed basal layer of sapric peat. Peat bulk density (BD) ranges between 0.06 and 0.6 Mg m
3 (mostly between 0.1 and 0.3 Mg m
3), and particle density (PD) between 1.39 and 2.22 Mg m
3; being 0.2 Mg m
3 and 1.5 Mg m
3 the bibliography reference values (Fig. 4.10). The highest values correspond to sapric peats, as already indicated by Lynn et al. (1974), to transitions between the peat and the mineral soil, and to minerogenic peats with a high proportion of mineral components, though in this case the change in particle density is greater than the change in bulk density (Van Lierop, 1981; PontevedraPombal, 1995). Organic carbon content ranges between 15% and 57%, nitrogen between 0.1% and 1.7%, and sulfur between 0.2% and 2.6% (Fig. 4.10). The lowest concentrations occur in layers with higher particle density. Although the evolution of organic matter in these mires is very slow due to effects of the low oxygen availability and oligotrophy on microbial activity (Gorham, 1995), a minimum level is maintained through time (Damman, 1988), which, in the ombrotrophic mires, results in an accumulation of C with depth/age. Although the degree of humification varies between mires, a greater degree of decomposition of organic matter occurs in the basal transition to the more mineralized layers where high ratios have been measured between the carbon extracted in pyrophosphate to total carbon (Cp/Ct), whereas the overlying peat samples show very low Cp/Ct ratios (Fig. 4.11). In obrotrophic mires, pH values range between 3.2 and 4.9, indicative of very acid to acid conditions (Fig. 4.12), a fact coherent with elevated rainfall, acid substrata, the acidifying effect of mosses (Motzkin, 1994) and organic acids formed during the decomposition of the organic matter (autoacidification). Exceptionally, pH is slightly

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Figure 4.11. Distribution of total carbon (left) and pyrophosphate carbon–total carbon ratios (Cp/Ct) (right) in ombrogenic and minerogenic mires.

Figure 4.12. pH values in representative ombrogenic and minerogenic mires.

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Figure 4.13. Effective cationic exchange complex (eCEC), basic cations sum (S) and aluminum (Al) differences between representative ombrogenic (left) and a minerogenic (central and right) mires.

acid to neutral in mires fed by waters draining carbonate rocks (Bran˜a de Lamelas (BLA); Table 4.5). Effective cation exchange capacity (eCEC) varies between 4 and 20 cmolc kg
1. The sequence of exchangeable cations is Mg2+,Ca2+4Al3+4Na+4K+ in the upper peat layers of ombrotrophic mires and Al3+4Mg2+, Na+4Ca2+4K+ in the deeper ones; in minerotrophic mires the most frequent sequence is Al3+4Ca2+4Mg2+4Na+4K+ (in BLA Ca is more abundant than Al). The abundance of Mg in the ombrotrophic, coastal, mires is due to the effect of marine aerosols, whereas the dominance of Al in the minerogenic ones is a consequence of the composition of the waters draining acid soils. Despite these generalizations, many mires show changes in the vertical profiles of the exchangeable cations (Fig. 4.13). Cation exchange capacity (CEC) measured at pH 7 is, as expected, much higher (24–165 cmolc kg
1); the largest differences with eCEC at soil pH occur in the most acid and decomposed peats.

Classification The classification of mires is particularly difficult due to several issues. These include the subjectivity of the methods used to determine the degree of peat decomposition, fiber content, the difficulty to estimate clay content in a material dominated by organic matter and the unavoidable heterogeneity of the methodology applied to study peat properties. With these limitations in mind and based on the World Reference Base for Soil Resources (FAO-WRB, 1998), mires from Galicia can be classified as histosols (Table 4.6), mainly as fibric, sapric and terric histosols; although thionic histosols are also represented. Using the USDA Soil Taxonomy (Soil Survey Staff, 1998), most of these soils are classified as fibrist (boro-, sphagno- and medi-fibrists), but also as hemists (boro-, sulfi- and medi-hemists) and saprists (boro, sulfi- and medi-saprists).

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Table 4.6. World reference base for soil resources classification (FAO–WRB, 1998) and soil taxonomy, Soil Survey Staff (1998), of representative Galician mires. Mire

Location

Tremoal da Gan˜idoira Bran˜a de Agolada

Serra do Xistral Serra dos Ancares Serra dos Ancares ManzanedaQueixa Serra dos Ancares Montes d Buio

Bran˜a de Lamelas Bran˜a dos Xuncos Bran˜a de Porto Ancares Bran˜a do Buio

Tremoal Cadramo´n Chao de Lamoso Campa da Cespedosa Lagoa de Lucenza Manzaneda II Manzaneda I Tremoal Pena da Cadela Veiga do Tremoal Tremoal Penido Vello Poza da Lagoa Maior Bran˜a de Queixa Borralleiras da Cal Grande Bran˜a de Sua´rbol

Serra do Xistral Serra do Xistral Serra dos Ancares Serra do Caurel ManzanedaQueixa ManzanedaQueixa Serra do Xistral A Toxiza Serra do Xistral Serra dos Ancares Sierra de Queixa Montes do Buio Serra dos Ancares

Altitude (m asl)

Age (14C yr BP)

720

6895750 (130 cm)

1230

3390740 (215 cm)

1280

3090735 (165 cm)

1580

–

1580

10,6507170 (195 cm)

620

7725750 (315 cm)

1040




1039

8785730 (415 cm)

1415

2070725 (95 cm)

1440

17,390790 (700 cm)

1700

–

1630

–

900

4600780 (185 cm)

700

5080740 (220 cm)

790

4070750 (245 cm)

1330

10,3707210 (265 cm)

1600

–

620

4660770 (230 cm)

1080

1250725 (70 cm)

Classification Terric Histosol Medisaprist Thionic Histosol Sulfihemist Sapric Histosol Borofibrist Terric Histosol Medifibrist Saprinc Histosol Sulfihemist Terric/fibric Histosol Medihemist/ Medifibrist Fibric Histosol Sphagnofibrist Fibric Histosol Sphagnofibrist Sapric Histosol Borosaprist Terric Histosol Borohemist Fibric Histosol Sphagnofibrist Fibirc Histosol Sphagnofibrist Fibric Histosol Sphagnofibrist Terric Histosol Sphagnofibrist Fibric Histosol Sphagnofibrist Sapric Histosol Borofibrist Fibric Histosol Borofibrist Fibric Histosol Medihemist/ Medifibrist Sapric Histosol Sulfisaprist

Galician mires: geochemical archives of environmental changes The ombrotrophic mires of NW Spain have been successfully used as archives of environmental changes. The relationship between the accumulation of heavy metals and human activities was studied in detail for Pb and Hg. For Pb, for example, it has been found that anthropogenic pollution may have begun some 2800–3000 years ago,

Mountain mires from Galicia (NW Spain)

103

Figure 4.14. Chronology of Pb enrichments of Tremoal do Penido Vello mire (PVO, Serras Septentrionais). The EF Pb (enrichment factor) is normalized to the average Pb/Ti ratio of pre-anthropogenic peat samples (filled area); dashed line illustrates variation through time of the Pb isotopic ratio (206P/ 207Pb) (age in 14C yr BP).

as a result of the production and trade of ternary bronzes with the Phoenicians in southern Spain (Fig. 4.14; Martı´ nez Cortizas et al., 1997, 2002; Pontevedra-Pombal, 2002). The beginning of a pervasive atmospheric Pb pollution is dated to the Iron Age and attained its maximum during the Roman period, with intensities on the order of five times the pre-pollution times fluxes. The records also show local pollution episodes during medieval times, coeval with forest decline and soil erosion, suggestive of intensive human impacts on the landscape (Martı´ nez Cortizas et al., 2005). The largest Pb enrichments occur near the surface of the profiles and are related to the industrial period. The analysis of the isotopic composition of the Pb in Galicia also supports the chronology of Pb fluxes (Martı´ nez Cortizas et al., 2002). Lead from pre-pollution times (43000 years) shows high values of the 206Pb/207Pb ratios (1.275), whereas the Roman period and the uppermost samples of the profiles show much lower ratios (206Pb/207Pb, respectively of 1.182 and 1.157), indicating anthropogenic pollution by mining/smelting activities and, in more recent times, the combustion of leaded gasoline. This trend is consistent with that described for other areas of the world (Settle and Patterson, 1980; Shotyk et al., 1988; Norton et al., 1990; Nriagu, 1996; Steinnes et al., 1997; Dunlap et al., 1999; Renberg et al., 2000; Weiss et al., 2002). Mercury anthropogenic pollution was also traced back to 2400–2500 14C yr BP, and the chronology of the changes have been found to be consistent with the history of mining and metallurgy of this element in the Iberian Peninsula (Fig. 4.15; Martı´ nez Cortizas et al., 1999). But Hg accumulation and the thermal-lability of the accumulated Hg were also found to depend on climatic conditions at the time of deposition. In short, cold climates promoted an enhanced accumulation of low thermal-lability Hg, whereas warm climates favored a lower accumulation and higher thermal-stability

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Figure 4.15. Average anthropogenic/natural Hg ratio values through time. (a) Total Hg (HgT) and the natural component of Hg accumulation (HgNAT, gray area). (b) Anthropogenic Hg (HgANT) at Tremoal do Penido Vello mire (PVO, Serras Septentrionais) and the main prehistoric and historic phases of the Hg exploitation in Spain (age in 14C yr BP).

Hg. Based on this behavior it was possible to reconstruct the changes in paleotemperatures during the last 4000 years, which show prominent cold (Neoglaciation and Little Ice Age) and warm periods (warm Roman period, medieval warm period). The record also shows an increase of temperature during the 20th century. The records of other elements where also investigated. Table 4.7 shows the accumulation of several elements at the Pena da Cadela (PDC) bog during the last 4000, 500 and 300 years (Martı´ nez Cortizas et al., 2002). For the last 4000 years, Fe is the element showing a higher accumulation (some 3000 kg ha
1) followed by Br and Ti (300–430 kg ha
1). Zinc, Pb, Sr, Zr and Cr have net accumulations of 10–40 kg ha
1, whereas the rest of the elements show values between 2 and 8 kg ha
1. Thus, at this coastal mire the long-term accumulation is dominated by the most abundant lithogenic elements that arrived to the mire as dust, and by those of marine origin (like Br). Nevertheless, on short times scales, Zn, Mn, Pb, As, Cr, Ni and Cu show larger proportions of the total accumulation, suggesting a relationship with the increasing impact of industrialization.

Conclusions Strong oceanic influence, high precipitation and wide geomorphologic heterogeneity, linked to Quaternary evolution, allowed the development of more than 10,000 ha of

Mountain mires from Galicia (NW Spain)

105

Table 4.7. Accumulation of several elements (kg ha
1) at the Pena da Cadela (PDC) bog (Serras Septentrionais, Lugo) during the last 4000, 500, and 300 years. The bold numbers indicate the relative proportion of the total accumulation. Element

4000 years

500 years

Fe Br Ti Zn Pb Sr Zr Cr Mn Cu Ni As Rb Se Ga Y

2874.0 431.8 329.0 36.0 22.6 15.1 13.4 9.8 7.5 6.9 5.9 5.8 4.1 2.9 2.6 2.4

908.0 64.1 82.3 28.2 16.5 4.6 3.5 3.2 5.7 2.4 1.9 4.1 1.5 0.6 1.1 0.8

300 years 0.32 0.15 0.25 0.78 0.73 0.30 0.26 0.33 0.76 0.35 0.32 0.71 0.36 0.21 0.42 0.33

548.0 32.3 41.1 20.1 12.2 2.5 1.6 2.5 4.9 1.6 1.2 2.6 1.9 0.3 0.7 0.5

0.19 0.07 0.12 0.55 0.54 0.16 0.12 0.26 0.65 0.23 0.20 0.45 0.22 0.11 0.28 0.19

mires ecosystems in Galicia (NW Spain). The peculiarities associated to their formation and evolution make these wetlands important in Europe, some having developed at extreme geographic locations. For example, blanket bogs of Galicia have developed at the southwestern limit of this type of mire in Europe. Galician mires have formed during the Late Quaternary in three main phases. During the Preboreal and the Boreal (11,000–8500 years ago) minerogenic mires were formed, in the mid Atlantic (7800–7100 years ago) both minerogenic and ombrogenic mires developed and between the mid Atlantic and the beginning of the sub-Boreal (6000–2000 years ago) an expansion of ombrogenic mires and a stabilization of ecologic conditions of the minerogenic ones took place. Average peat thickness is 2 m for minerogenic and 3 m for ombrogenic mires, with growth rates between 0.2 and 0.7 mm yr
1; the higher values occur in ombrogenic mires. Mean dry mass accumulation is 95 g m
2 yr
1 and carbon accumulation averages 37 g C m
2 yr
1. Minerotrophic mires accumulate more mass but less carbon than ombrotrophic mires. Bulk density decreases from the basal layers to the surface of the mire; the highest values are found in sapric organic materials, at transitions to the mineral sediment, or in layers of minerogenic mires with high ash content. The mires of Galicia are mainly acid mires (pH 2.5–5.0), with the lowest pH in the superficial peat layers. Cation exchange capacity ranges between 5 and 18 cmol(+) kg
1, with Mg2+ dominating in ombrotrophic mires and Al3+ in the minerotrophic ones. In ombrotrophic mires C increases with depth/age, whereas in the minerotrophic C content decreases with depth as the degree of evolution of the organic matter. Ombrotrophic mires of Galicia are true archives of chemical proxies of environmental evolution, and are involved in the cycles of many trace elements (like As, Hg,

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Pb, Zn) of environmental concern due to their potential toxicity. These archives reveal that anthropogenic atmospheric pollution in Spain dates back to at least 2500–2800 years ago, the Roman period representing a pollution climax in preindustrial times. During the last 500 years atmospheric pollution has increased abruptly, although some elements are showing symptoms of moderate decreases in recent atmospheric fluxes (as for example Pb).

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