64
but numbers were too low to produce percentage values. Species number (average 3
species) was very low. This interval can be interpreted as the anthropogenic deposit
introduced during the agricultural reclamation of the area observed in historical aerial photography from 1946 (Fig. 2.10). The following 18 cm (DI 2) were characterized by low and upward-increasing numbers of foraminiferal tests (mean 230 tests/50
g) dominated by E. macrescens (average 70%) and T. inflata (average 28%). Species
number (average 3 species) was very low. This interval represents the environmental
regeneration process occurred as tidal water invaded the formerly occupied salt
marsh, progressively improving the living conditions for benthic foraminifera. The
last 23 cm (DI 1) showed high numbers of foraminiferal tests (mean 3416 tests/50
g), dominated by E. macrescens (average 67%) and T. inflata (average 27%).
Haplophragmoides wilberti (average 4%) and A. mexicana (average 2%) appeared
as secondary forms. Species number (average 5 species) was low and slightly higher
than in the lower DIs. This interval with the highest foraminiferal abundance reflects
the stabilization of the environment and characterizes the regenerated salt marsh.
Sand content was very low throughout the core (average 1%) that was mainly
composed of mud (Table 2.3; Fig. 2.11). Therefore, the area represented a low
energy environment since the human occupation period (DI 3) until the regenerated
high-marsh environment (DI 1) was developed. A slight increase in sand content
was observed from the bottom to the top of the core, which could be related to the
entrance of sand-rich estuarine water as regeneration took place.
2.6.1.2 Dating the Sedimentary Record
The
210
Pb xs profile showed a typical exponential decay (Fig. 2.12), apparently allowing the use of radiometric dating techniques. However, application of the CF model
(Sanchez-Cabeza and Ruiz-Fernández 2012) indicated that by 1946 the salt marsh
was already regenerated. This seriously contradicts aerial photography that shows
agricultural lands in this area at that time (Fig. 2.10: top left). This contradiction can
be explained by the occurrence of sediment mixing during land reclamation that
causes the misinterpretation of the
210
Pb xs inventory for age determination. In fact,
historical photographs reflect how areas that appeared cultivated in the late 1940s
were already undistinguishable from other surrounding salt-marsh areas by the
early 1970s. On the contrary, the
137
Cs activity profile seems to offer a more reliable
chronological benchmark determined by the peak indicative of 1963 located at 23
cm, in the limit between DI 2 (salt marsh in regeneration) and DI 1 (regenerated salt
marsh). This age determination is also in agreement with Pb-derived chronology, as
this metal exhibited maximum values (period 1965–1975) slightly above the bottom
of DI 1. Pb values were normalized with Al to be comparable to previous regional
studies, even though sand content barely changed throughout the core. Consequently,
the regeneration process (DI 2) of the Carasa salt marsh took around 10
years (between the late 1940s and the early 1960s) based on aerial photography, the
137
Cs peak, and the Pb/Al profile. Sedimentation rates were calculated taking into
account the thickness of the DI of interest and the age of its upper and lower limits.
A. García-Artola et al.
but numbers were too low to produce percentage values. Species number (average 3
species) was very low. This interval can be interpreted as the anthropogenic deposit
introduced during the agricultural reclamation of the area observed in historical aerial photography from 1946 (Fig. 2.10). The following 18 cm (DI 2) were characterized by low and upward-increasing numbers of foraminiferal tests (mean 230 tests/50
g) dominated by E. macrescens (average 70%) and T. inflata (average 28%). Species
number (average 3 species) was very low. This interval represents the environmental
regeneration process occurred as tidal water invaded the formerly occupied salt
marsh, progressively improving the living conditions for benthic foraminifera. The
last 23 cm (DI 1) showed high numbers of foraminiferal tests (mean 3416 tests/50
g), dominated by E. macrescens (average 67%) and T. inflata (average 27%).
Haplophragmoides wilberti (average 4%) and A. mexicana (average 2%) appeared
as secondary forms. Species number (average 5 species) was low and slightly higher
than in the lower DIs. This interval with the highest foraminiferal abundance reflects
the stabilization of the environment and characterizes the regenerated salt marsh.
Sand content was very low throughout the core (average 1%) that was mainly
composed of mud (Table 2.3; Fig. 2.11). Therefore, the area represented a low
energy environment since the human occupation period (DI 3) until the regenerated
high-marsh environment (DI 1) was developed. A slight increase in sand content
was observed from the bottom to the top of the core, which could be related to the
entrance of sand-rich estuarine water as regeneration took place.
2.6.1.2 Dating the Sedimentary Record
The
210
Pb xs profile showed a typical exponential decay (Fig. 2.12), apparently allowing the use of radiometric dating techniques. However, application of the CF model
(Sanchez-Cabeza and Ruiz-Fernández 2012) indicated that by 1946 the salt marsh
was already regenerated. This seriously contradicts aerial photography that shows
agricultural lands in this area at that time (Fig. 2.10: top left). This contradiction can
be explained by the occurrence of sediment mixing during land reclamation that
causes the misinterpretation of the
210
Pb xs inventory for age determination. In fact,
historical photographs reflect how areas that appeared cultivated in the late 1940s
were already undistinguishable from other surrounding salt-marsh areas by the
early 1970s. On the contrary, the
137
Cs activity profile seems to offer a more reliable
chronological benchmark determined by the peak indicative of 1963 located at 23
cm, in the limit between DI 2 (salt marsh in regeneration) and DI 1 (regenerated salt
marsh). This age determination is also in agreement with Pb-derived chronology, as
this metal exhibited maximum values (period 1965–1975) slightly above the bottom
of DI 1. Pb values were normalized with Al to be comparable to previous regional
studies, even though sand content barely changed throughout the core. Consequently,
the regeneration process (DI 2) of the Carasa salt marsh took around 10
years (between the late 1940s and the early 1960s) based on aerial photography, the
137
Cs peak, and the Pb/Al profile. Sedimentation rates were calculated taking into
account the thickness of the DI of interest and the age of its upper and lower limits.
A. García-Artola et al.