73
2.7 Regional Overview
Foraminifera and sand present a clear temporal pattern indicative of the agricultural
reclamation and the subsequent environmental regeneration of salt marshes in
norther Spain: agricultural soils can be recognized by the nearly absence of foraminifera; salt marshes under environmental regeneration show increasing amounts
of foraminifera and sand, together with very high sedimentation rates linked to the
inundation by tidal waters carrying sandy sediments; and, regenerated salt-marsh
environments present abundant foraminifera.
These recently regenerated salt marshes challenge the use of
210
Pb for dating
sediments. In fact, agricultural activities cause sediment mixing, which can create a
hiatus, a misinterpretation of background levels, and an underestimation of the
210
Pb xs inventory. This is common in recently regenerated environments where the
agricultural occupation period coincides with the timeframe of the
210
Pb dating
method (~100–120 years). In fact, radiometric studies are based upon undisturbed
and continuous sediment sequences. Although these conditions are not always met
in previously reclaimed salt marshes, it is common to observe that some
210
Pb xs
profiles show a typical exponential decay with depth (Fig. 2.12), pointing out that
sedimentary disturbances may be often difficult to detect exclusively from
210
Pb xs
profiles. However, the
210
Pb dating method can be successfully applied in areas
where human intervention was prior to the 1900s and the
210
Pb xs inventory was not
anthropogenically manipulated (García-Artola et al. 2016). In recently disturbed
areas, we recommend a combination of multiple proxies such as
137
Cs and heavy
metals for age estimation. Furthermore, historical aerial photography is an essential
age-validation tool.
A correct age determination allows the estimation of sedimentation rates, which
were very high (usually 14–18 mm year
−1
; up to 40 mm year
−1
at lower elevation
environments) during the environmental regeneration process of formerly occupied
salt marshes. This process occurred in less than 10 years, mainly between the 1950s
and the 1960s. In the Plentzia estuary it took place slightly later, during the 1960s,
and in some locations of the Santoña estuary even before the 1940s (see GarcíaArtola et al. 2016 for additional regional examples shown in Fig. 2.1). The elevated
sedimentation rates are responsible for the fast environmental regeneration and
respond to the ability of salt marshes to reach equilibrium with the tidal frame. Salt
marshes situated lower in the tidal frame tend to accrete faster than those located at
higher elevations (van Wijnen and Bakker 2001; Temmerman et al. 2004; Goodman
et al. 2007). As an example, the Isuskiza core presents sedimentation rates exceeding
any previous records due to the low elevation of the reclaimed land. As the occupied
salt marsh was invaded by tidal waters, a tidal flat was formed (clear dominance of
calcareous foraminiferal species) that quickly gained enough elevation during the
regeneration process (at around 40 mm year
−1
) to become the modern regenerated
salt marsh. This regenerated environment differs from a natural salt marsh in terms
of organic matter content (Santín et al. 2009) and vegetation diversity that might take
longer to fully recover (Garbutt and Wolters 2008; Chang et al. 2016).
2 Recent Agricultural Occupation and Environmental Regeneration of Salt...
2.7 Regional Overview
Foraminifera and sand present a clear temporal pattern indicative of the agricultural
reclamation and the subsequent environmental regeneration of salt marshes in
norther Spain: agricultural soils can be recognized by the nearly absence of foraminifera; salt marshes under environmental regeneration show increasing amounts
of foraminifera and sand, together with very high sedimentation rates linked to the
inundation by tidal waters carrying sandy sediments; and, regenerated salt-marsh
environments present abundant foraminifera.
These recently regenerated salt marshes challenge the use of
210
Pb for dating
sediments. In fact, agricultural activities cause sediment mixing, which can create a
hiatus, a misinterpretation of background levels, and an underestimation of the
210
Pb xs inventory. This is common in recently regenerated environments where the
agricultural occupation period coincides with the timeframe of the
210
Pb dating
method (~100–120 years). In fact, radiometric studies are based upon undisturbed
and continuous sediment sequences. Although these conditions are not always met
in previously reclaimed salt marshes, it is common to observe that some
210
Pb xs
profiles show a typical exponential decay with depth (Fig. 2.12), pointing out that
sedimentary disturbances may be often difficult to detect exclusively from
210
Pb xs
profiles. However, the
210
Pb dating method can be successfully applied in areas
where human intervention was prior to the 1900s and the
210
Pb xs inventory was not
anthropogenically manipulated (García-Artola et al. 2016). In recently disturbed
areas, we recommend a combination of multiple proxies such as
137
Cs and heavy
metals for age estimation. Furthermore, historical aerial photography is an essential
age-validation tool.
A correct age determination allows the estimation of sedimentation rates, which
were very high (usually 14–18 mm year
−1
; up to 40 mm year
−1
at lower elevation
environments) during the environmental regeneration process of formerly occupied
salt marshes. This process occurred in less than 10 years, mainly between the 1950s
and the 1960s. In the Plentzia estuary it took place slightly later, during the 1960s,
and in some locations of the Santoña estuary even before the 1940s (see GarcíaArtola et al. 2016 for additional regional examples shown in Fig. 2.1). The elevated
sedimentation rates are responsible for the fast environmental regeneration and
respond to the ability of salt marshes to reach equilibrium with the tidal frame. Salt
marshes situated lower in the tidal frame tend to accrete faster than those located at
higher elevations (van Wijnen and Bakker 2001; Temmerman et al. 2004; Goodman
et al. 2007). As an example, the Isuskiza core presents sedimentation rates exceeding
any previous records due to the low elevation of the reclaimed land. As the occupied
salt marsh was invaded by tidal waters, a tidal flat was formed (clear dominance of
calcareous foraminiferal species) that quickly gained enough elevation during the
regeneration process (at around 40 mm year
−1
) to become the modern regenerated
salt marsh. This regenerated environment differs from a natural salt marsh in terms
of organic matter content (Santín et al. 2009) and vegetation diversity that might take
longer to fully recover (Garbutt and Wolters 2008; Chang et al. 2016).
2 Recent Agricultural Occupation and Environmental Regeneration of Salt...