DOI: 10.25921/0zsr-9m55
title: NOAA/WDS Paleoclimatology - Bear Lake, Utah-Idaho 25,000 Year Geochemical and Mineralogical Data

description: Sediments deposited over the past 220,000 years in Bear Lake, Utah and Idaho, are predominantly calcareous silty clay, with calcite as the dominant carbonate mineral. The abundance of siliciclastic sediment indicates that the Bear River usually was connected to Bear Lake. However, three marl intervals containing more than 50% CaCO3 were deposited during the Holocene and the last two interglacial intervals, equivalent to marine oxygen isotope stages (MIS) 5 and 7, indicating times when the Bear River was not connected to the lake. Aragonite is the dominant mineral in two of these three high-carbonate intervals. The high-carbonate, aragonitic intervals coincide with warm interglacial continental climates and warm Pacific sea-surface temperatures. Aragonite also is the dominant mineral in a carbonate-cemented microbialite mound that formed in the southwestern part of the lake over the last several thousand years. The history of carbonate sedimentation in Bear Lake is documented through the study of isotopic ratios of oxygen, carbon, and strontium, organic carbon content, CaCO3 content, X-ray diffraction mineralogy, and HCl-leach chemistry on samples from sediment traps, gravity cores, piston cores, drill cores, and microbialites.

Sediment-trap studies show that the carbonate mineral that precipitates in the surface waters of the lake today is high-Mg calcite. The lake began to precipitate high-Mg calcite sometime in the mid–twentieth century after the artificial diversion of Bear River into Bear Lake that began in 1911. This diversion drastically reduced the salinity and Mg2+:Ca2+ of the lake water and changed the primary carbonate precipitate from aragonite to high-Mg calcite. However, sediment-trap and core studies show that aragonite is the dominant mineral accumulating on the lake floor today, even though it is not precipitating in surface waters. The isotopic studies show that this aragonite is derived from reworking and redistribution of shallow-water sediment that is at least 50 yr old, and probably older. Apparently, the microbialite mound also stopped forming aragonite cement sometime after Bear River diversion. Because of reworking of old aragonite, the bulk mineralogy of carbonate in bottom sediments has not changed very much since the diversion. However, the diversion is marked by very distinct changes in the chemical and isotopic composition of the bulk carbonate.

After the last glacial interval (LGI), a large amount of endogenic carbonate began to precipitate in Bear Lake when the Pacific moisture that filled the large pluvial lakes of the Great Basin during the LGI diminished, and Bear River apparently abandoned Bear Lake. At first, the carbonate that formed was low-Mg calcite, but ~11,000 years ago, salinity and Mg2+:Ca2+ thresholds must have been crossed because the amount of aragonite gradually increased. Aragonite is the dominant carbonate mineral that has accumulated in the lake for the past 7000 years, with the addition of high-Mg calcite after the diversion of Bear River into the lake at the beginning of the twentieth century.
description: Piston cores were collected in 1996 from three localities in Bear Lake 
using the University of Minnesota, Limnological Research Center's (UMN-LRC) 
Kullenberg coring system (Dean et al., 2006).   Core BL96-1 is 5 m long 
from a water depth of 50 m.  Core BL96-2 is 4 m long from a water depth 
of 40 m.  Core BL96-3 is 4 m long from a water depth of 30 m.  
Overlapping cores provide a 25,000-year record.  

Unknown amounts of sediments were missing from the tops of the piston cores, 
so surface sediments (up to 50 cm) were collected with a gravity corer 
in 1998 (Dean et al., 2007).  Core BL98-4 is 17 cm m long from a water depth 
of 30 m.  Core BL98-6 is 20 cm long from a water depth of 30 m. Core BL98-9 
is 30 cm long from a water depth of 40 m.  Core BL98-10 is 36 cm long from a 
water depth of 40 m.  Core BL98-12 is 38 cm long from a water depth of 30 m.  
Core BL98-13 is 36 cm long from a water depth of 30 m.         

Piston cores and gravity cores were collected at five localities at the 
northern end of the lake in 2002.  Geochemical and mineralogical data 
were collected on two of the piston cores and gravity cores from the 
same localities.  Core BL02-3PC is 475 cm long and BL02-3GC is 15 cm long, 
both from a water depth of 40 m.  Core BL02-4PC is 380 cm long and BL02-4GC 
is 40 cm long, both from a water depth of 30 m.

The data set includes files of carbon data, X-ray diffraction (XRD) mineralogy, 
carbon- and oxygen-isotope data on bulk carbonate, Sr isotope data on bulk 
carbonate, and HCl-leach chemistry.   Not all data were collected on 
sediments from all cores.

Bear Lake: 
42°0'N, 111°20'W, 1805m elev.