Southern Baja California Peninsula Research Articles

Transport of benthic diatoms across the continental shelf off southern Baja California Peninsula

Material collected with a time-series sediment trap moored in San Lázaro Basin, near the NE Pacific upwelling zone off Bahía Magdalena, Baja California Sur (Mexico), from August to November 1996 and December 1997 to June 1998, revealed temporal changes in the flux of benthic diatoms. Fluxes were highest during fall and spring. Lateral transport of benthic diatoms from the coastal zone seems to be an important phenomenon at this site. The species assemblage includes components found mainly in mangrove environments of Bahía Magdalena, as well as nearby rocky coastlines, indicating that locally the northward counter current or coastal gyres carry living cells, and possibly other particles of lagoon origin, across the continental shelf.

Stress protein expression is related to tail loss in two species of iguanid lizards

During the spring of 1999, two species of iguanid lizards were captured in southern Baja California peninsula, Mexico. Blood was obtained from the tail to check for the presence of blood parasites in smears and stress proteins in cells. Levels of HSP70- and HSP60-like proteins from Sceloporus licki Van Denburgh, 1895 and Petrosaurus thalassinus (Cope, 1863) were analyzed with Western blot using antisera to human HSP60 and bovine HSP70. The potential effects of sex, tail regeneration, and haemoparasites on the expression of these proteins were also investigated. Both species differ significantly in stress protein levels and infection by haemoparasites. Petrosaurus thalassinus show the lower stress protein levels and the higher proportion of infection. Although blood parasites apparently affect the condition of P. thalassinus, stress protein levels are not significantly related with haemoparasites or condition. However, S. licki lizards showing tail regeneration have lower levels of HSP60-like protein. A negative relationship exists between the length of the regenerated part of the tail and the level of HSP60-like protein for S. licki. Based on what is known of the function of HSP60, down-regulation of the protein in blood cells may be linked to reallocation of energies to other tissues more active metabolically.

The western screech-owl and habitat alteration in Baja California: a gradient from urban and rural landscapes to natural habitat

We studied the western screech-owl (Otus kennicottii) in the desert of the southern Baja California peninsula to determine its status and habitat selection and whether it benefits from moderate human-caused habitat changes. Based on the response to tape-recorded call broadcasts, western screech-owls were more abundant in undisturbed vegetation than in human-altered habitat. In man-made environments, more owls responded in rural than in urban areas; indeed, they were practically absent in urban areas. In natural areas, a total of 1.6 owls/km was estimated in the nonbreeding season and 2.7 owls/km in the breeding season. The most important habitat features selected in natural areas were greater shrub cover in the strata containing shrubs 1–2 and 2–3 m high and greater canopy cover of trees >5 m high. In rural areas adjacent to cultivated areas, owls were present in areas with the greatest cover of shrubs 1–3 m high and a higher density of shrubs. The western screech-owl also occurs at lower densities in rural areas than in natural areas. Towns appear to be avoided in the arid desert of Baja California Sur. Western screech-owls may be still able to persist in rural areas that occupy an intermediate position on the gradient from a functional natural system (desert vegetation) to a completely altered ecological system, urbanized areas.

Evolution of the margin of the Gulf of California near Loreto, Baja California Peninsula, Mexico

The Gulf of California is a prime example of a young oblique-divergent plate boundary. This type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings. A complexity in the Gulf of California is that the modern oblique rifting was preceded by a stage of orthogonal rifting. We have used extensive mapping as well as stratigraphic, geomorphic, and structural analysis to determine the history of deformation near Loreto on the southern Baja California peninsula during formation of the Gulf of California. Our data support the suggestion that middle to late Miocene (protogulf stage, 12 to ca. 6 Ma) orthogonal rifting was overprinted by transtensional structures during Pliocene to Quaternary time, though we cannot closely define the time of change. We further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults. The Loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned, down-tothe-east extensional monoclines and normal faults. These structures lie along, or as far as 4 km in front (east) of, a steep, 1000‐ 1600-m-high Main Gulf Escarpment that defines the western topographic margin of the rift zone. The Loreto segment is further defined by a rise in elevation of the Main Gulf Escarpment of up to 1 km and an ;500‐800 m increase in structural relief of prerift strata in the escarpment from the segment boundaries to the center of the segment. Structural analysis of secondary faults of the Loreto segment shows that fault populations that are known to be Pliocene to Quaternary in age are mixed normal and dextral-normal faults with a bulk extension direction of west-northwest‐eastsoutheast (2808‐1008). In contrast, fault populations that cut only prerift, Oligocene‐middle Miocene volcanic and sedimentary rocks are mainly normal faults with northeast-southwest to east-northeast‐ west-southwest bulk extension directions that average 2458‐658. These faults are interpreted to be late Miocene in age and formed during protogulf orthogonal rifting. Thus, the Pliocene faults record a major change of extension direction of ;358 from latest Miocene to Pliocene time, compatible with the onset of oblique rifting in the Gulf of California. Oblique-divergent overprinting is mainly expressed as the Loreto fault and Loreto basin in the northern part of the segment. The Loreto fault and basin evolved with major fault reorganizations and partial basin inversion. Many observations suggest that the Pliocene Loreto fault was linked to two nascent transform faults in the gulf and that Carmen Island rotated ;358‐408 clockwise within the complex fault system.

Comparison between the Chile and Mexico triple junction areas substantiates slab window development beneath northwestern Mexico during the past 12–10 Myr

When combined with the Miocene–Recent volcanic record of Baja California, a parallel drawn between the Chile and Mexico triple junction areas substantiates slab window development beneath northwestern Mexico during the past 12–10 Myr. The slab-free zone manifestations challenge the notion that ridge subduction has not occurred beneath the southern Baja California peninsula. The geochemically distinctive rocks from the Santa Clara volcanic field of west–central Baja California, including coeval adakites and niobium-enriched basalt, are commonly inferred to signal partial melting of the subducting plate at shallow depths and relatively high temperatures, before slab dehydration occurs. Such PT conditions for slab melting have only been observed in association with spreading-ridge subduction. We propose that slab window development beneath southern Baja California and mainland Mexico (30° to 18°N) resulted from subduction of the East Pacific rise.

Two new Marina (Leguminosae) from the southern Baja California peninsula, Mexico

Marina victoriae and M. brevis are newly described species from the southern Baja California peninsula, Mexico. The former is a perennial herb locally abundant in the eroded hillsides at the foothills of Sierra de la Victoria, where an oak woodland and a dry tropical forest meet. The latter is an elusive ephemeral inhabiting a narrow strip of land between the mangroves and the desert scrub on an island in the Gulf of California. These two new species belong to the series Chrysorrhizae, a group native to the Gulf of California basin.

Stratigraphy and geochronology of the Comondú Group near Loreto, Baja California sur, Mexico

Abstract Upper Oligocene to Middle Miocene volcanic and sedimentary rocks in the Loreto region, Baja California Sur, are widely exposed and make up the Comondu Group as redefined here following McFall. The Comondu Group is part of a volcanic arc and forearc basin that formed along the northwestern margin of Mexico. Regional to detailed scale mapping, stratigraphic analysis, and geochronology in a 10–20-km-wide and 70-km-long belt from the gulf escarpment to the coast near Loreto reveal three main units in the Comondu Group and a composite thickness of ∼1.5–2 km. (1) The lower clastic unit (∼30–19 Ma) contains 200–300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin. (2) The middle breccia and lava flow unit (∼19–15 Ma) is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows. (3) The upper lava flow and breccia unit (∼15–12 Ma) consists almost entirely of up to ∼600 meters of andesite lava flows a few kilometers west of Loreto that are part of a composite volcano. North and south of Loreto for up to 25 km, the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers. The three units of the Comondu Group in the Loreto area correlated with the Salto, Pelones, and Ricason formations of the Bahia Concepcion area. The lower clastic unit of this study is time equivalent to the marine San Gregorio, El Cien, and Isidro formations on the western and southern Baja California peninsula. The middle and upper units of the Loreto area are the proximal equivalent to the type section of the Comondu Group near the village of Comondu. During deposition of the lower part of the Comondu Group, the arc lay to the east on mainland Mexico in the Sierra Madre Occidental, and then migrated to the position of the Gulf of California at about 25 Ma. At ∼19 Ma the proximal part of the arc migrated abruptly >50 km westward to approximately the position of Carmen Island and at 15 Ma the arc once again stepped west forming eruptive centers along the coast near Loreto until ∼12 Ma. Also at 15 Ma, the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment.

Sedimentology and rare earth element geochemistry of phosphatic sediments from Sebastián Vizcaíno Bay, Baja California, Mexico

Surficial sediments rich in apatite pellets were recovered from an 80‐m‐deep area that runs parallel to the eastern coast of Vizcaino Bay, Baja California. This area is subject to intense seasonal upwelling, the sediments presenting elevated phosphate (P2O5), organic carbon (Corg), and carbonate concentrations. The pellets are moderately well sorted, concentrated mainly in the 3ø size class within a poorly sorted sediment matrix. Abundant nodular apatite and phosphatized bone fragments were found in only one of the pellet‐bearing samples. This sample has a P2O5concentration of over 20%, the ≥ — 2.5ø to 0.5ø grain size classes alone being formed of up to 32% P2O5. The high proportion of fragmented nodules and the presence of volcanic rock pebbles indicate that at least part of this deposit is allochthonous. The apatite pellets have a flat to concave‐downward heavy rare earth (HREE) shale normalized pattern and, except for La, are depleted in light rare earth elements (LREE). The close resemblance between the HREE patterns and the relatively high La concentration in the pellets of Vizcaino Bay, and the onland phosphorites of the southern Baja California Peninsula, suggest similar depositional histories for these deposits. No significant Ce anomalies were observed, but a negative Eu deviation is common in all of the pellet‐bearing sediments. The depletion of LREE in the pellets of Vizcaino Bay may indicate the preferential removal of these elements by weathering processes.

Taxonomy of the genus Amphiroa (Corallinales, Rhodophyta) in the southern Baja California Peninsula, Mexico

Abstract Studies of the genus Amphiroa (Corallinales, Rhodophyta) along the southern Baja California Peninsula, Mexico, were undertaken to determine how many species were present and to assess the characters that have been or could be used in species delimitation. After analysis of 800 specimens from 60 localities, we conclude that five species are present: A. valonioides Yendo, A. misakiensis Yendo, A. vanbosseae Lemoine, A. rigida Lamouroux and A. beauvoisii Lamouroux. Amphiroa brevianceps Dawson and A. magdalensis Dawson are considered heterotypic synonyms of A. misakiensis. Characters used here for the first time to delimit species of Amphiroa include shape of gametangial conceptacle chambers, ontogeny of the pore of the gametangial conceptacle, the type of spermatangia present, the position of gonimoblast filaments and the form of the cortical cells. Characters used in species delimitation in Amphiroa are discussed in detail and a key to the species found in southern Baja California is provided.