42
consequences of future relative sea-level rise? The answers to these questions lie in
exploration of the past and to achieve that we need to generate relative sea-level
reconstructions.
We need a proxy for sea-level. That is, a physical, biological, or chemical marker
of how high relative sea level was in the past that is preserved for thousands of years
and stands in for the direct instrumental tide gauges. Fortunately, North Carolina
has such a proxy in abundance: salt marshes and the organisms that live on and in
them. On low-energy, temperate coasts, salt marshes mark the transition between
the terrestrial and marine realms. As a consequence of occupying this unique position, salt marshes exhibit a strong environmental gradient encompassing marine,
brackish, and freshwater conditions driven by the frequency and duration of inundation by tidal water. The plants that live on salt marshes are adapted to spending at
least some amount of time submerged by, or exposed to, saltwater.
Plants that live at lower elevations on the salt marsh must be able to survive relatively long and frequent intervals of submergence. In North Carolina, the plant best
suited to this habitat is Spartina alterniflora (smooth cord grass). Plants that live at
higher elevations on the salt marsh need only survive relatively short and perhaps
infrequent intervals of submergence and this environment in North Carolina is dominated by Juncus roemerianus (black needle rush). Salt-marsh plants in North
Carolina (and elsewhere) are therefore a sea-level proxy. As a side note on black
needle rush, it is one of the few plants to actively take up silica that it then uses to
construct a very pointy tip (hence the name needle). If you visit a North Carolina
salt marsh be sure to wear long sleeves and eye protection and be prepared to spend
the day getting painfully needled and the evening removing tiny splinters where the
silica-rich tips of the needles broke off in your skin.
A nice example of just how predictable the relationship between plant species
and elevation is on salt marshes comes from Boston. In the late nineteenth and early
twentieth centuries, surveyors working in Boston sometimes used a reference level
called “marsh datum”, because they recognized that the high salt-marsh zone (as
identified by the characteristic plants that live there) in particular had a very predictable and robust relationship to the mean high water tide level and that the marsh
itself was a strikingly level plane. At that time Boston had considerably more intact
salt marsh than it does today, so it was often more convenient for surveyors to level
to a nearby salt marsh rather than running a level line to a known benchmark.
Go one step further and examine microscopic (and basically immobile) organisms living on the salt-marsh surface and it becomes clear that they too form distinctive assemblages that live at specific elevations. The most commonly used group of
organisms in this role are foraminifera (single-celled organisms with a shell that live
in saltwater environments; Fig. 3.4).
If relative sea level rises, a salt marsh responds by accumulating sediment on the
surface to maintain a constant elevation through time. The best place to reconstruct
relative sea level is to find the most boring, flat salt marsh imaginable; one that successfully maintained its elevation with minimal disturbance for hundreds to thousands of years. In this game, “Looks like nothin’s gonna change, Everything seems
to stay the same” is exactly the type of place we want to find! But how would you
A. C. Kemp and B. P. Horton
consequences of future relative sea-level rise? The answers to these questions lie in
exploration of the past and to achieve that we need to generate relative sea-level
reconstructions.
We need a proxy for sea-level. That is, a physical, biological, or chemical marker
of how high relative sea level was in the past that is preserved for thousands of years
and stands in for the direct instrumental tide gauges. Fortunately, North Carolina
has such a proxy in abundance: salt marshes and the organisms that live on and in
them. On low-energy, temperate coasts, salt marshes mark the transition between
the terrestrial and marine realms. As a consequence of occupying this unique position, salt marshes exhibit a strong environmental gradient encompassing marine,
brackish, and freshwater conditions driven by the frequency and duration of inundation by tidal water. The plants that live on salt marshes are adapted to spending at
least some amount of time submerged by, or exposed to, saltwater.
Plants that live at lower elevations on the salt marsh must be able to survive relatively long and frequent intervals of submergence. In North Carolina, the plant best
suited to this habitat is Spartina alterniflora (smooth cord grass). Plants that live at
higher elevations on the salt marsh need only survive relatively short and perhaps
infrequent intervals of submergence and this environment in North Carolina is dominated by Juncus roemerianus (black needle rush). Salt-marsh plants in North
Carolina (and elsewhere) are therefore a sea-level proxy. As a side note on black
needle rush, it is one of the few plants to actively take up silica that it then uses to
construct a very pointy tip (hence the name needle). If you visit a North Carolina
salt marsh be sure to wear long sleeves and eye protection and be prepared to spend
the day getting painfully needled and the evening removing tiny splinters where the
silica-rich tips of the needles broke off in your skin.
A nice example of just how predictable the relationship between plant species
and elevation is on salt marshes comes from Boston. In the late nineteenth and early
twentieth centuries, surveyors working in Boston sometimes used a reference level
called “marsh datum”, because they recognized that the high salt-marsh zone (as
identified by the characteristic plants that live there) in particular had a very predictable and robust relationship to the mean high water tide level and that the marsh
itself was a strikingly level plane. At that time Boston had considerably more intact
salt marsh than it does today, so it was often more convenient for surveyors to level
to a nearby salt marsh rather than running a level line to a known benchmark.
Go one step further and examine microscopic (and basically immobile) organisms living on the salt-marsh surface and it becomes clear that they too form distinctive assemblages that live at specific elevations. The most commonly used group of
organisms in this role are foraminifera (single-celled organisms with a shell that live
in saltwater environments; Fig. 3.4).
If relative sea level rises, a salt marsh responds by accumulating sediment on the
surface to maintain a constant elevation through time. The best place to reconstruct
relative sea level is to find the most boring, flat salt marsh imaginable; one that successfully maintained its elevation with minimal disturbance for hundreds to thousands of years. In this game, “Looks like nothin’s gonna change, Everything seems
to stay the same” is exactly the type of place we want to find! But how would you
A. C. Kemp and B. P. Horton