Sunday, May 20, 2018

Coastal-sabkha strand line near San Felipe, Baja California Pt 2

This post is the second part of the types of organisms found (Aug. 1976) on the extreme western limit of a coastal sabkha along the west side of the northern Gulf of California, in Baja California, Mexico. Part 1 concerned the location of the sabkha, as well as images of mostly terrestrial lifeforms and swimming crabs. Part 2 concerns the mollusks found in a 20-m wide band of drift material restricted to the strand line.



Bivalves were extremely common and consisted only of single valves, many of which were unbroken. Some of the valves occurred in localized concentrations, whereas others were broken and scattered across the mud-cracked surface. Isolated large fragments of wood (as shown above) were also present.




Broken bivalves adjacent to mud cracks.




The infaunal (burrowing) bivalve Mulinia modesta Carpenter,  shown above, was formerly referred to as Mulinia coloradoensis Dall. Before the dams across the Colorado River were built, this endemic (found only in this region) bivalve was  the most abundant mollusk living on the Colorado Delta. Valves of this species occur in huge numbers in about 10 to 13 km to the east, where they have been subsequently washed out of the tidal flats and now form shelly beaches, beach ridges (= cheniers), and shoals of the delta. At the strand line, there are twice as many right-hand valves and there are left-hand valves. This is evidence of selective sorting by waves. The valves found at the strand line were missing also the outermost perisostracum layer (a horny protective layer) and were bleached.


Chionista californiensis? (Broderip) is a common infaunal clam living intertidally on mudflats, as well as offshore.



              
Nassarius howardae? Chace was a moderately common gastropod in the strand-line assemblage. Nassarius gastropods are active scavengers, which burrow horizontally just below the surface of the bottom. Nassarius gastropods are commonly tidal-flat dwellers. The specimens found at the strand line were slightly worn, and the tips of the shells were commonly missing.


Phalium (Semicassis) centiquadrata (Valenciennes) is a gastropod known to live in the sand of very shallow-marine waters. This species was rare (only three found) in the strand-line assemblage. The shells were the following features: protoconch (earliest part of the tip), nodes on the shoulder of the last whorl, outer lip, and some of the more delicate features along the inner side of the aperture (opening of the shell). In general, these shells were considerably worn, and the shell was much thinner than normal.


Ficus ventricosa (Sowerby) is a shallow-subtidal (offshore) gastropod. This species was very rare (only one  found) in the strand-line assemblage. The protoconch was present, but the outer lip is broken


Concerning the taphonomy (what happens after a lifeform dies), the mudflat-dwelling Mulinia and Chione shells, and probably the Nassarius shells,  were likely transported reworked a few km and concentrated by the tidal currents and storm waves at the strand line. The nearshore-marine shells of Phalium (Semicassiscentiquadrata and Ficus ventricosa were likely transported as much as 10 to 13 km distance by a tidal bore and/or storm waves, which moved across the low-relief sabkha surface. The absence of a barrier-island system in this area allows for unrestricted flooding, which enhances currents capable of transport the remains of marine organisms.  

Given enough burial and time, strand-line remains will become part of the sequence of coastal tidal-flat sediments consisting of muddy siltstone and claystone, with lenticular beds of gypsum and halite, as well as interfingering lobes of alluvial debris. This sequence has all the characteristics of an arid inland basin, except for the intertidal and marine shells. Normally, one would not expect marine shells to be transported 10 to 13 km (6 to 8 mi.) and to be in such good shape, but the evidence shown above proves otherwise.


Mixed assemblages of marine and terrestrial organisms are important in distinguishing between ancient examples of coastal sabkhas and continental sabkhas.

Monday, May 7, 2018

Coastal-sabkha strand line near San Felipe, Baja California Pt 1



On August 8, 1976, while on a field trip to the San Felipe area, northern Baja California, I and my colleagues spent a day looking at the biology of a strand-line (shoreline) assemblage at the westernmost limit of a large coastal sabkha (uppermost super tidal-flat environment). This assemblage, which is shown below, was collected at a locality 3 km (2 mi.) east of Mexico Highway 5 and about 60 km north of San Felipe.

The locality is on the west side of the Colorado River Delta. The delta has undergone significant environmental change since the building of dams across the river and subsequent diversion of water. The influx of freshwater into the delta region was greatly reduced and, therefore, there has been increased salinity in the region.



Location of the sabkha (white area) and locality area, plotted on a Google Earth (2018)-generated image, is shown above. Spring tides (new and full Moon phases) in the northern Gulf of California (present day) are known to be some of the highest in the world (and most dangerous). They can have a spring-tidal range of up to 10 m, with an accompanying tidal-bore front as much as 1.5 to 3 m high in an almost vertical wall of water moving at about 2 knots.




View due east (toward the ocean) of the salt-incrusted mudflat of the sabkha just east of the strand-line locality. The open waters of the Gulf of California were several kilometers (10 to 13 km) away, but standing water with salt covering the water occurred about 300 m east of the strand line. This low-lying sabkha usually floods in the summer during very high spring tides. The grooves are salt-encrusted tire tracks, made by off-road enthusiasts driving across the very muddy surface. This would have been a highly risky undertaking because of the high likelihood of getting stuck. 

Closeup of the moist, muddy ground in the immediate area of the strand-line locality. Pen is 13 cm length.



View of the August 8, 1976 strand-line locality. The week before (July 25–28) I visited the site, there was a spring tide (new moon), associated with high tides.
 


Remains of fauna and flora found along the strand-line zone (about 20 m wide) include scattered pieces of wood, gourds, beetles, bird eggs, a few nearly complete birds with feathers intact (not shown above), bones of the brown pelican [Pelecanus occidentals californicus, not shown here], land-mammal leg bones and jawbones, algal scum, whole fish (up to 20 cm long), whole crabs, single and broken valves of bivalves, gastropod shells, and also some glass floats used by commercial fishermen. The scale is 25 cm long.  The fauna and flora represent a mixture of some terrestrial life and abundant marine life, all confined to the 20-m wide zone. Immediately east of the strand line, the salty mudflat was essentially barren of lifeforms.

To the south of where the remains were found, a thin lobe of alluvium extended into the strand-line zone. The lobe was related to a flash flood, which transported small pebbles to large cobbles of igneous rocks (including pebbles of pumice), large wood fragments (see image below), and beetles into the strand-line zone. West of the strand line there was sparse chaparral vegetation (e.g., some ocotillo plants).

The following images show closeups of some of the strand-line biota. 
gourd with seeds inside (scale is 5 cm)


bird egg (30 mm length)


Cryptoglossa verrucosa ground beetles. They were young adults to adults and complete with legs, mandibles, and antennae. Their soft parts had been mummified.



Mugil cephalus, a mullet fish (encrusted by salt residue). This fish occurs in coastal estuaries and lagoons throughout the region. Specimens found at the strand line were mostly complete and well preserved, with skin and fins present. The specimens ranged from tiny juveniles to large adults. 

Callinectes bellicosus, a "swimming crab" (largest specimen approximately 12 cm wide). This crab lives in mudflat channels and nearshore areas. The specimens found at the strand line showed excellent preservation: Most  are complete with legs, chelipeds (claws), numerous sharp spines along the edge of the carapace, and both young adult and adult specimens were found. None showed any signs of abrasion. Some were bleached. Locally, concentrations of only the chelipeds were present.


Large glass floats (each one about 16 cm diameter), used by commercial fishermen for their fishing nets.

PART 2 OF THIS POST = THE NEXT POST

note: All the pictures of the remains were taken 42 years ago, as Kodachrome slides, which I scanned and converted into digital images.