Chronologic Framework

The integration of biostratigraphy, radiocarbon dating, and oxygen isotope stratigraphy provides a chronologic framework for the late Quaternary evolution of the Rio Grande fluvial/deltaic/slope fan system. Biostratigraphic information is obtained by analyzing fluctuations in the occurrence of two species of planktonic foraminifera, Globorotalia menardii and Globorotalia inflata. These fluctuations define Ericson-Wollin zones. Three accelerator mass spectrometer radiocarbon dates, acquired by analyzing planktonic foraminifers (Globogerinoides ruber), provide further information for the chronologic framework.

Planktonic foraminifers
Foraminifer counts were made as part of preliminary paleowater depth estimates. In general, 300 or more specimens were counted per sample, when enough foraminifers were available. The counts were done on the 250 to 355 (m size fraction. The foraminifera within each sample were subdivided into planktics, porcelaneous benthics, arenaceous benthics, and perforate benthics. This methodology was used to gain a preliminary understanding of major changes in paleoenvironments and therefore, to infer changes in paleowater depth. A rigorous quantitative biostratigraphic analysis involving further subdivision of the foraminifer species and statistical analysis would be required to gain a detailed understanding of the variations in paleowater depth experienced at the B-2 core locations.
Variations in Globorotalia inflata and Globorotalia menardii complex occurrences are used to define Ericson-Wollin zones.

Zone Z: characterized by occurrence of G. menardii plexus
Z - Y Boundary: marked by disappearance of G. menardii and the appearance of G. inflata
Zone Y: joint occurrence of both G. menardii and G. inflata; the planktonic foraminifer Globorotalia truncatulinoides is present in the interval and it switches from right to left coiling directions (right coiling foraminifers are associated with warm water masses, and left coiling forms with cool water masses).
Y - X Boundary: marked by change in dominance from the G. inflata to the G. menardii complex
Zone X: characterized by occurrence of G. menardii
X - W Boundary: Two candidates, Xa and Xb.
Xa: marked by a change from G. menardii complex to a zone of no planktonic foraminifers and few benthonic foraminifers.
Xb: dominated by porcelaneous benthic foraminifers with a small number of planktonic foraminifers
Zone W: dominated by G. inflata

Radiocarbon Dating
Three foraminiferal samples (Globogerinoides ruber) from core B-2 were dated using an accelerator mass spectrometer. The samples consisted of 1200 to 2000 clean planktonic foraminifers each.

Sample Number 2462-B2-46 2462-B2-35 210-2462-B2-19
Depth (m) 14.02 10.67 5.79
Age 45,140 ( 1300 45.120 ( 980 4490 ( 70

Oxygen Isotope Stratigraphy
An oxygen isotope curve was generated using 133 planktonic foraminiferal samples (Globogerinoides ruber) from core B-2 and used as a proxy for sea level and a chronologic tool. The oxygen isotope curve was interpreted by comparing it with two other oxygen isotope curves derived form analyses of Globogerinoides ruber from cores within the Gulf of Mexico that had good chronologic frameworks (DSDP site 619 in the Pigmy Basin offshore Louisiana from Williams and Kohl, 1986; and TR-126-23 in the southwestern Gulf of Mexico from Williams, 1984). There was close agreement between the three oxygen isotope curves.

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