LATE PLEISTOCENE AIRPORT LANE FOSSIL SITE, LA GRANDE, NE OREGON.

Microfossils

Abstract

 Diatoms (Aulacoseira; Navicula; Nitzchia; and Rossithidium); sponge spicules (Ephydatia fluviatilis); and grass (Gramineae), juniper (Juniperus), and pondweed (Potamatogen) pollen in the silty fine sand inside and surrounding the mammoth tusks at the Airport Lane fossil site suggest deposition in shallow streams and adjacent floodplains at the distal margins of the late Pleistocene La Grande outwash fan.  The presence of western hemlock (Tsuga heterophylla) pollen at the site suggests that western hemlock was more abundant in the nearby hills during the late Pleistocene than it is today.  The creeks in the area have cut down ~18 m since the deposition of the sediments at the Airport Lane fossil site in the late Pleistocene.

Introduction

The grayish-brown silty fine sand inside and surrounding the mammoth tusks at the Airport Lane fossil site contains rare freshwater diatoms and sponge spicules and very rare pollen grains.  These were examined by making smear slides of the silt fraction of the sediment and examining the microfossils at 400x magnification using a Leitz Laborlux Pol petrographic microscope.

The Airport Lane Fossil Site Diatoms

The diatoms in the sediments at the Airport Lane fossil site (Figure 1) include Aulacoseira, Navicula, Nitzchia, and Rossithidium (previously known as Achnanthes).  Aulacoseira is a freshwater planktic diatom; Navicula and Nitzchia are freshwater motile diatoms; and Rossithidium is a freshwater attached diatom.  Nitzchia is an indicator of water with a high content of dissolved ions (Bradbury, 1988).

Figure 1. Diatoms found in the sediment inside and surroundig the Airport Lane mammoth tusks: A. Aulacoseira; B. Navicula; C. Nitzchia; and D. Rossithidium.

                  The diatoms in the Airport Lane fossil site sediments can be classified as:

                  Kingdom                Proctotista

                  Phylum                  Bacillariophyta

                                    Class        Bacillariophyceae

                                                      Order      Achnanthales

                                                                        Family Achnanthaceae

                                                                        Genus     Rossithidium Rukhtiyarova & Round 1996

                                                      Order      Bacillariales

                                                                        Family    Bacillariaceae

                                                                        Genus     Nitzchia Hassell 1845

                                                      Order      Naviculales

                                                                        Family    Naviculaceae

                                                                        Genus     Navicula Bory de Saint-Vincent 1822

                                    Class        Coscinodiscophyceae

                                                      Order      Aulacoseirales

                                                      Family    Aulacoseiraceae

                                                      Genus     Aulacoseira Thwaites 1848

                  All of these diatoms have been reported in the Columbia River (U.S. Department of the Interior, 1966) and, with the exception of Rossithidium, in the sediments of the Grande Ronde Valley (Van Tassell and others, 2001).

The Airport Lane Fossil Site Sponge

 The sponge spicules in the Airport Lane fossil site sediments belong to the species Ephydatia fluviatilis (Figure 2). This species of sponge is common in in the sediments of the GrandeRonde Valley (Van Tassell and others, 2001).

Figure 2. Sponge spicule (Ephydatia fluviatilis) found in the sediment inside and surrounding the Airport Lane mammoth tusks.

                  The Airport Lane fossil site sponge spicules may be classified as:

                  Kingdom                Animalia

                  Phylum                   Porifera

                  Class                          Demospongiae

                  Order                        Haplosclerida

                  Suborder               Spongillina

                  Family                       Spongillidae

                  Genus                       Ephydatia

                  Species                     Ephydatia fluviatilis (Linnaeus, 1759)

Ephydatia fluviatilis is an indicator of water temperatures of 12°-14°C, water depths less than 1.5 m, and high ion concentrations (Bright, 1982).

The Airport Lane Fossil Site Pollen

 Pollen grains identified in the Airport Lane fossil site sediments (Figure 3) include grass (Gramineae), juniper (Juniperus), western hemlock (Tsuga heterophylla), and pondweed (Potamogeton).

Figure 3. Pollen from the sediment inside and surrounding the mammoth tusks at the Airport Lane fossil site: A. Grass (Gramineae); B. Juniper (Juniperus); C. Western hemlock (Tsuga heterophylla); and D. Pondweed (Potamogeton).

Western hemlock (Tsuga heterophylla) has been found in 4400-year old lake sediments in the Wallowa Mountains to the east of the Grande Ronde Valley (Beck, 1996).  The only known modern stand of western hemlock in eastern Oregon is a small refugial population at an elevation of 1951 m in the Indian Creek drainage on the east side of the Grande Ronde Valley (Oregon Plant Atlas, 2011).

Discussion

The diatoms and sponge spicules in the silty fine sands within and around the mammoth tusks at the Airport Lane fossil site suggest deposition in shallow streams and adjacent floodplain environments at the distal margin of the late Pleistocene La Grande outwash fan.  Pondweed (Potamatogen) grew along the banks of the creeks and lakes in the area.  The site is ~18 m above the elevation of Catherine Creek, ~0.8 km to the southeast of the site.  Juniper trees (Juniperus) may have dotted the landscape.  Grasses and pondweed provided a food source for mammoths, bison, and other herbivores.  Western hemlock (Tsuga heterophylla) pollen in the sediments suggests that stands of western hemlock were much more extensive on the slopes surrounding Grande Ronde Valley than at present.

Conclusions

 Diatoms (Aulacoseira; Navicula; Nitzchia; and Rossithidium) and sponge spicules (Ephydatia fluviatilis) in the silty fine sands at the Airport Lane fossil site suggest deposition in shallow streams and adjacent floodplains at the distal margins of the late Pleistocene La Grande outwash fan.  Vegetation in the area included grasses (Gramineae), juniper (Juniperus), and pondweed (Potamatogen).  The grasses and pondweed provided food for the large herbivores that occupied the area.  The creeks in the area have down cut ~ 18 m since the late Pleistocene.

References Cited

Beck, A.P., 1996, 4400 years of vegetation change at Twin lakes, Wallowa Mountains, northeast Oregon: M.S. thesis, Washington State University, 51 p.

Bradbury, J.P., 1988, Diatom biostratigraphy and the paleolimnology of Clear Lake, Lake County, California, in Sims, J.D., ed., Late Quaternary climate, tectonism, and sedimentation in Clear Lake, northern California Coast Range: Geological Society of America Special Paper 214, p. 97-129.

Bright, R.C., 1982, Paleontology of the lacustrine member of the American Falls lake beds, southeastern Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 597-614.

Oregon Plant Atlas, 2011, http://ww.oregonflora.org/site/Plants.php?source=13176.

U.S. Department of the Interior, 1966, A guide to the common diatoms at water pollution surveillance system stations: Cincinnati, Ohio, Federal Water Pollution Control Administration, 0-220-505, 98 p.

Van Tassell, J.; Ferns, M.; McConnell, V., and Smith, G.R., 2001, The mid-Pliocene Imbler fish fossils, Grande Ronde Valley, Union County, Oregon, and the connection between Lake Idaho and the Columbia River: Oregon Geology, v. 63, no. 3, p. 77-84, 89-96.