Cation, Anion Concentrations Along Lake Sediment/Water Interface


As part of the Long Term Ecological Research (LTER) project in the McMurdo Dry Valleys of Antarctica, a systematic aqueous geochemical sampling program has been undertaken. A series of water samples have been collected and analyzed for major ion chemistry by ion chromatography. The concentrations of ions cover a wide range of total dissolved solids from fresh to hypersaline lake waters. This dataset shows concentrations of lithium, sodium, potassium, magnesium, calcium, iron, chlorine, bromine, and SO4 found along the sediment/water interface of Taylor Valley lakes.

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Dedicated sampling equipment for each lake has been important for maintaining the integrity of the samples. For example, the range of Cl- concentration in the investigated lakes varies by more than three orders of magnitude. This carryover between samples becomes a potential problem. Carryover can also be a potential problem during analysis, and therefore analytical blanks are run to monitor this.

The initial sample processing was carried out in field laboratories. A 100-ml sample aliquot was filtered for each anion and cation sample. All samples were filtered through 0.4-microm Nucleopore filters, usually within 12h of collection. The cation samples were filtered into acid-washed polyethylene bottles which had been rinsed with DI water, while the anion samples were filtered into DI-washed bottles. The cation samples were acidified by adding 0.5% (v/v) of concentrated HCl. Quality control was maintained by carefully rinsing all filtration apparatus with DI between samples and by processing filtration blanks which were later run as samples.

The majority of water chemistry samples were returned to the Crary Laboratory, McMurdo Station, Antarctica, for analysis. Some of the samples were returned to the laboratory at the University of Alabama for analysis owing to time constraints during the field season.

DX-300 ion chromatographic system was used for the major ion analyses. The system included a gradient pump module, high-pressure injection valve with a 25-microm sample loop, a Dionex conductivity detector (CDM-3) advanced computer interface and automated sampler. The timed events and data collection were controlled by the Dionex AI-450 chromatography software for Windows. The same ion chromatographic system was used but was switched back and forth between anion and cation configurations.

For the anions, a Dionex Ionpac AS4A-SC analytical column (250x4mm I.D.) and AG4A-SC guard column (50x4mm I.D.) and AG4A-SC guard column (50x4mm I.D.) were used along with an Anion Self-Regenerating Suppressor-1. The eluent was 1.8 mM Na2CO3-1.7 mM NaHCO3. The gradient pump flow-rate was 2 ml/min and the background conductivity was ca. 16 uS.

For the cations, the Dionex Ionpac CS12 analytical column (250x4mm I.D.) and CG12 guard column (50x4mm I.D) were used with a Cation Self-Regenerating Suppressor-1. The eluent was 0.020 M methanesulfonic acid. The eluent flow-rate was 1.0 ml/min and the background conductivity was ca. 200-250 nS.

The stock standard solutions used for a typical batch of samples from the lakes are specified below:

  Na+     	   	100		  Cl-			100
  K+		 	 20		SO4(2-)			100
  Mg(2+)	 	 50		   F-			 10
  Ca(2+)	 	 50		  Br-			  2
  Li+           	  1	

Appropriate dilutions of the stock standards were used to prepare a range of standards for calibration. Owing to the high salt concentrations in many of the samples, dilutions were made before the samples were run. Dilutions ranged from 1:2 for Lake Hoare surface water up to 1:6000 for the Lake Bonney deep water. The samples were diluted by serial dilution, using plastic microbeakers and adjustable pipettors.

Replicate and duplicate samples were run daily. Usually, each sample was injected twice and samples from each batch were run in duplicates to check the precision of the dilutions. In almost every case, the relative standard deviation of the duplicates was less than 1%, even with dilutions of 1:6000. In addition, analytical blanks and filtration blanks were analyzed to check the quality control.

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Data and Metadata was moved to DEIMS by Inigo on 2015.Data sets were submitted to the data manager by Kathy Welch at the University of Alabama. The raw data files listed under 'file name' are the names of the original files submitted, which are stored in the /data1/data/lakes/lakechem/ directory on INSTAAR's Unix system. These files are ascii text files. Upon arrival at INSTAAR, the data manager added variables for lake, date, and location description. Kathy Welch could specify which season and lake the samples were drawn from. However, more specific locations, sample dates, and description of sampling methods will need to be updated by Peter Doran.The file was imported into Microsoft Access version 2.0 on INSTAAR's Unix system, and can currently be found there. The file was then exported in ascii, comma delimited text and MS-DOS text (table layout) to present on the MCM LTER web site. Both of these files are linked to this web page above.

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Data contained in these files has been subjected to quality control standards imposed by the investigator. The user of this data should be aware that, while efforts have been taken to ensure that these data are of the highest quality, there is no guarantee of perfection for the data contained herein and the possibility of errors exists. If you encounter questionable data, please contact the MCM LTER data manager corrected or qualified. Thus, these data may be modified and future data will be appended.
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