Geology Reference
In-Depth Information
Radiometric dating of the sediment cores was used to
provide an age structure for
adjoining dune and beach sands, basin subsidence linked to
dissolution, and a complex history of expansion and con-
traction of the basin (Semeniuk
2007
) (Fig.
1
).
ve wetland basins (Semeniuk
2007
). Pollen from the main species in the wetlands was
studied biostratigraphically. The species studied were: the
herb
Centella asiatica
(L.) Urb., the sedges
Baumea artic-
ulata
(R. Br.) S. T. Blake,
Typha
(L.) sp., the paperbarks
Melaleuca teretifolia
Endl.,
M. rhaphiophylla
Schauer,
M.
viminea
Lindley assemblage,
M. cuticularis
Labill., the
wetland peripheral grasstree
Xanthorrhoea preissii
Endl., the
wetland peripheral sedge
Isolepis nodosa
(Rottb.) R. Br. and
the sea couch
Sporobolus virginicus
(L.) Kunth.
Comparison of pollen diversity and abundance against
wetland age structure indicated that, except for the grasstree
Xanthorrhoea preissii
and local sedge
Isolepis nodosa
, most
wetland species were present over the interval of 4,500 years
BP to the present in the middle-to late-Holocene. However,
there were a number of other important patterns in the down
pro
3.2
Pollen and Charophyte Record
Wetland basin 161, in its 4,500-year old stratigraphy,
records a variable pollen and charophyte biostratigraphy
(Semeniuk et al.
2006a
,
b
).
In the pollen record, thirteen key and recurring species of
wetland plants were selected for the study (Semeniuk
2007
),
viz.,
Baumea articulata
,
Baumea juncea
,
Typha orientalis
,
Typha domingensis
,
Isolepis nodosa
,
Juncus kraussii
,
Lep-
idospermum gladiatum
,
Centella asiatica
,
Sporobolus vir-
ginicus
,
Melaleuca rhaphiophylla
,
M. teretifolia
,
M.
viminea
,
M. cuticularis
, and
Xanthorrhoea preissii
(a wet-
land margin inhabitant). There is
le abundance of wetland pollen taxa and their inter-
basin variation at isochronous levels such as: a lack of
continuity down pro
uctuation in numbers of
pollen of these species down the stratigraphic column
(Fig.
2
), which Semeniuk (
2007
) interpreted as recording
climate change due to Earth-axis Precession (resulting in a
general increase in regional humidity) and recording the
intra-basinal hydrochemical adjustments as one vegetation
assemblage succeeded another (essentially, freshwater
assemblages alternated with brackish water assemblages
re
fl
uctuations in
numbers of pollen taxa that were continuous; lack of cor-
relation in timing of the peak pollen numbers between sep-
arate basins; variable total composition at the same isochron
level from wetland to wetland; variable total composition of
wetland pollen at different ages within the same wetland; the
association of pollen species with sediment types; increases
and decreases of wetland margin pollen taxa in the down
pro
le for some species;
fl
ecting both ecologically driven changes and cyclic cli-
mate changes).
Biostratigraphically, charophyte gyrogonites in wetland
161 largely mirror the palynological record. The gyrogonites
of the two charophyte species are present down the strati-
graphic column, viz.
Chara globularis
and
Chara
fl
le composition; and the recent appearance of
I. nodosa
and
X. preissii
within the last circa 1,500 years (Semeniuk
2007
). The patterns of pollen derived from wetland vegeta-
tion in individual wetlands suggest that the ancestral distri-
bution and abundance of plant assemblages in the Becher
wetlands was a function of intra-basin environmental chan-
ges caused by wetland evolution. In contrast, pollen derived
from upland vegetation exhibited continuity down pro
brosa
.
These species inhabit freshwater to brackish water (1
9%
TDS) so that, in this context, they are not sensitive indicators
of wetland salinity in the Becher Point system. Their con-
tribution to environmental reconstructions is that they rep-
resent periods of marked carbonate production, which can be
related to climate, such as early summer drying of wetlands
(and hence a trend towards aridity), and increased water
temperature, when end-of-winter surface waters are warmed
by insolation.
The pollen and charophyte record has important use
geologically in the reconstruction of climate history,
hydrology, and hydrochemistry for individual wetlands and
for the region. The combined pollen and charophyte record
suggest the following patterns (Semeniuk
2007
):
1.
-
le,
suggesting that delivery of upland pollen has been largely
consistent, though variable in abundance and composition
from basin to basin, and being exogenic, it has not re
ected
(hydrochemical, edaphic or hydroperiod) environmental
conditions within the wetlands.
fl
3.1
Wetland Basin 161 Stratigraphy
Wetlands on the Becher Point Cuspate Foreland generally are
small geomorphic and hydrologic features, generally <100 m
in size. As noted above, the wetlands of the beach ridge plain
are graded in age from oldest to the east and youngest to the
west. The oldest wetland is basin 161 (Fig.
1
c, d) that pro-
vides a wealth of geological features and information at the
small scale and microscale. It records the thickest accumu-
lation of sediments, the diagenetic effects of past and ongoing
dissolution along its basal
the pollen record shows
uctuating numbers of the var-
ious species in response to intra-basin hydrological and
hydrochemical patterns;
fl
2.
the pollen record shows that the grasstree
X. preissii
and
the sedge
I. nodosa
, appear in the past 1,500 years,
related to the general increase in climate humidity;
3.
the species indicators of wet conditions
Typha
and
Baumea
broadly show relatively wet conditions in lower
interface with underlying and
