1.3.1D
ISTRIBUTIONS AND PALEOENVIRONMENTAL RECONSTRUCTIONMicrofossils such as pollen and foraminifera are widely utilized in intertidal paleoenvironmental reconstructions because their modern intertidal zonal distributions are tied to changes in elevation with respect to the tidal frame (e.g., Scott and Medioli, 1978; Gehrels, 1994; Horton et al., 1999; Engelhart et al., 2007). Figure 1.2 shows this relationship. Because different taxa of a particular microfossil group (e.g., foraminifera) can be related to specific tidal ranges, microfossils are useful as sea-level indicators. The correlation between a sea-level indicator and a
specific tidal range is known as indicative meaning (Shennan, 1986; van de Plassche, 1986). There are two defining parts to indicative meaning: the indicative range (IR) is the full elevational range within which the sea-level indicator can be found within the modern intertidal zone (e.g., MHW to MLW) and the midpoint in this range is the reference water level (RWL), which is often a tidal datum (e.g., MTL). This quantifiable relationship is fundamental to paleoenvironmental and paleoelevation reconstructions because the same assemblages preserved in Holocene intertidal stratigraphic sequences can then be related to their respective paleoenvironments and therefore, paleoelevation (Gehrels, 1999; Edwards and Horton, 2000; Hughes et al., 2002; Sawai et al., 2004a).
Pollen was one of the first microfossils used to identify changes in paleoenvironments (e.g., Godwin, 1940) and is still the most widely used reconstructive proxy for coastal studies (Tooley, 1978; Shennan, 1982; Sun et al., 1999; Hughes et al., 2002; Willard et al., 2003; Willard et al., 2005; Yulianto et al., 2005). The pollen assemblage of a sample is indicative of the coastal vegetation, which varies with changes to the inundation period, salinity, and pH of the environment (Erdtman, 1952; Brush and DeFries, 1981).
Figure 1.2. Foraminiferal distributions relative to tidal datums. Population relative abundances for each species are shown schematically with colored lines:
Jadammina macrescens (red), Trochammina inflata (blue), Tiphotrocha comprimata
(green), Miliammina fusca (pink), Quinqueloculina tenagos and Elphidium
mexicanum (orange). Tidal datums, relative to mean tide level: Highest astronomical tide (HAT), mean high water (MHW), mean low water (MLW), and lowest
However, there is a limitation in its usefulness as a proxy for paleoelevation: the distribution of intertidal marsh plants may be controlled by elevation, but the transport of the pollen is widespread (Traverse, 1988). Insects, water, and wind can transport pollen out of the environment in which the plant lives, which can decrease the precision of environmental reconstruction (Gehrels, 2007). Specifically, in estuarine environments the controls on the distribution of pollen are vast: the factors include, but are not limited to, the amount of pollen produced by the individual plant, the seasonality of flowering, the method of transport (wind vs. insect pollination), and the sediment/hydrological dynamics of the estuary (Brush and DeFries, 1981; Traverse, 1988). Despite these limitations, Brush and DeFries (1981) demonstrated in the Potomac estuary that the distribution of pollen preserved in the stratigraphic record was statistically comparable to the modern distributions because the morphology of pollen grains prevents them from being transported far away from the parent plant. This demonstrates the usefulness of pollen in reconstructing estuarine paleoenvironmental change.
Furthermore, pollen can have an advantage over other microfossils because it is the most robust microfossil, preserved in the stratigraphic record when other microfossils (e.g., foraminifera or diatoms) may have been destroyed. Its excellent preservation is due to pollen’s thick exine surface, which is made from sporopollenin (Zetzsche, 1932). Due to its complex biochemical structure, sporopollenin is resistant to nearly all forms of oxidation, and exposure to strong acids such as sulfuric acid, hydrochloric acid, and hydrofluoric acid. Some of the only known substances that can dissolve pollen are 2-aminoethanol and potassium hydroxide due to the process of hydrolytic (rather than oxidative) breakdown (Southworth, 1974; Bedinger, 1992).
This attribute is vital for paleoenvironmental reconstructions in tropical environments. Engelhart et al. (2007) confirmed that modern distributions of mangrove pollen are zoned relative to the tidal frame, thereby confirming their suitability for sea-level reconstructions. Quantitative statistical tests determined that pollen assemblages from this environment can be used to reconstruct sea level to a precision of ± 0.22 m (Engelhart et al., 2007).
Hughes et al. (2002) used pollen to reconstruct changes in paleoenvironment across lithologic boundaries and estimate the amount of coseismic subsidence due to the 1700 Cascadia earthquake in Tofino, British Columbia. Analysis of the pollen assemblages within the buried soil of the 1700 earthquake showed the soil was
enriched in the pollen of forest and upper-marsh plants such as Potentilla pacifica,
Achillea millefolium, and Gaultheria shallon, which are indicative of a forest-
edge/transition marsh environment. The overlying tsunami sand contained many of the same plant taxa as the buried soil, interpreted to be the result of the onshore mixing and deposition of pollen by the tsunami wave. The postseismic mud overlying the tsunami deposit was enriched in the pollen of low- to middle-marsh plants such
as Cyperaceae, Triglochin-type, Poaceaeand Chenopodiaceae, which areindicative of
a low to middle intertidal environment. A quantitative reconstruction from the pollen assemblages of the 1700 Cascadia earthquake by using modern pollen analogs and their related elevations yielded an average estimate of subsidence of 0.61 ± 0.3 m, which compared very well to the average estimate using foraminifera of 0.63 ± 0.3 m at the same location (e.g., Guilbault et al., 1996).
Foraminifera are excellent proxies for paleoenvironmental change because they can tolerate the conditions of nearly all marginal to marine environments and have lived
from the Cambrian to the present day, making them a useful proxy in nearly any marginal marine/marine environment through most of the history of multicellular life forms (Sen Gupta, 1999a). Indeed, the longest geologic records of climate change
are from deep-sea cores, reconstructed using the δ18O and δ13C signatures of
planktonic foraminifera that have been preserved in the stratigraphic record as far back as 65 Ma (Zachos et al., 2001). Foraminifera from deep-sea cores were instrumental in identifying and confirming the cyclical glacial/interglacial climate
pattern that defines the Quaternary (Hays et al., 1976). The δ18O proxy data from
foraminiferal tests revealed periodicities in global climate of 100,000 years, 41,000 years, and 23,000 years; the first concrete evidence for the extraplanetary climate forcings first proposed by Milankovitch in 1941 (Imbrie and Kipp, 1971; Hays et al., 1976; Imbrie et al., 1993).
Climate reconstructions spanning time periods of 100,000 years use planktonic foraminifera that live within the ocean water column and accumulate on the ocean floor. Reconstructions on the order of shorter timescales, (e.g., Holocene) use benthic foraminifera, species that live on or in the surface sediment of intertidal or marginal marine environments as proxies for paleoenvironmental change (Sen Gupta, 1999a). Benthic foraminifera are divided into morphologically distinct Orders that live in distinct marginal and fully marine environments (Sen Gupta, 1999b). For example, marsh foraminifera within the intertidal zone are dominated by
agglutinated taxa such as Trochammina inflata or Jadammina macrescens, which
construct their tests from clastic grains of the marsh substrate. Foraminifera that live
in subtidal to fully marine environments are dominated by species such as Ammonia
(Phleger, 1942; Phleger and Parker, 1951; Scott and Medioli, 1978; Culver and Buzas, 1980).
Scott and Medioli (1978; 1980) demonstrated that distinct intertidal benthic foraminiferal assemblages can be correlated with specific elevation ranges. For
example, a nearly mono-specific assemblage of Jadammina macrescens is associated
with the highest elevations of the marsh environment, while a mix of taxa including
Tiphotrocha comprimata and Trochammina inflata is indicative of lower intertidal
elevations (Scott and Medioli, 1978). The distribution of the mono-specific
assemblage of Jadammina macrescens is restricted to an elevational range of ± 0.05
m across the marsh surface, which leads to an accurate reconstruction of paleoelevation. Many common species of foraminifera that are used to reconstruct sea level are cosmopolitan, (the same species exist in the same environment in different areas of the world), making reconstructions of the same age from different
locations comparable (e.g., a marsh in coastal Oregon will be dominated by the same
foraminiferal taxa as a marsh environment in the United Kingdom (Figure 1.3).
Foraminifera can also be used to reconstruct and quantify sudden paleoenvironmental change and RSL, such as coseismic subsidence on an active coastal margin (e.g., Nelson et al., 2008). Methods in subsidence stratigraphy have revealed evidence for a chronology of earthquakes preserved in the coastal sediments of Cascadia over the late Holocene (Atwater, 1987; Clague, 1997; Kelsey et al., 2002; Nelson et al., 2006). Foraminifera, because of their relationship to the
Figure 1.3. The distribution of marsh foraminiferal assemblages is indicative of specific elevational ranges regardless of magnitude of the local tidal range or the geographical position of the study site (modified from Horton et al., in prep.).
tidal frame have been used to estimate the amount of coseismic subsidence related to the AD 1700 event in Cascadia. This was done by first establishing the relationship between changes in elevation in the intertidal zone to changes in the distribution of modern foraminifera across five transects in coastal Oregon. Generally, upland, high
marsh, and middle marsh elevations were dominated by Trochammina irregularis,
Balticammina pseudomacrescens, Haplophragmoides wilberti and Trochammina
inflata. The low marsh and tidal flat elevations were dominated by Miliammina fusca.
This relationship was then be applied to foraminiferal assemblages found in the stratigraphic sequences of the AD 1700 event using a transfer function technique, which matched the distinct foraminiferal assemblages found in the core to the correlated modern elevation (for information on transfer functions see Horton and Edwards, 2006). Changes in foraminiferal assemblages were identified between the preseismic buried soil horizon and the overlying post-seismic clastic horizon, yielding a change in elevation (coseismic subsidence) of 0.18 ± 0.20 m.
The identification of hurricane and tsunami deposits is initially defined by the presence of distinct sandy layers interrupting marsh or lacustrine depositional sequences (e.g., Atwater, 1987; Reed, 1989; Liu and Fearn, 1993; Nelson et al., 1996; Atwater and Hemphill-Haley, 1997; Shennan et al., 1999; Liu and Fearn, 2000; Donnelly et al., 2001a; Sedgwick and Davis, 2003; van de Plassche et al., 2004). The sedimentologic evidence for hurricane and tsunami deposits can be supported by microfossil data, which indicate abrupt changes in assemblages between the overlying deposit and the underlying material. Collins et al. (1999) confirmed that sand units located between continuous peat accumulation were deposited by Hurricane Hugo in South Carolina by identifying near-shore foraminifera
and re-worked benthic foraminifera within the deposit. Similar stratigraphic sequences have also been associated with tsunami deposits (e.g., Dawson et al., 1996; Dawson and Shi, 2000; Hawkes et al., 2007). Figure 1.4 illustrates the litho- and biofacies changes that occur before, during, and after tsunami deposition.
Macrofossils of intertidal and marine invertebrates can also be used to reconstruct changes in paleoenvironment and to reconstruct changes in relative sea level. One example is the vermetids, a group of gastropods that construct large reefs at low
intertidal elevations. A series of uplifted reefs built by the gastropod Dendropoma
petraeum were identified as evidence of seismic uplift in the eastern Mediterranean
over the last 3000 years. The reef constructing gastropod would be uplifted out of the environment with each coseismic event, creating a disruption in active reef building which could be recognized in the resulting reef stratigraphy (Pirazzoli et al., 1996). Macrofossils are suitable for sea-level reconstructions because their living populations can be related to a specific elevational range. Further, they construct
their shells from calcium carbonate, which can be sampled for 14C radiometric dating.