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National Geological and Geophysical Data Preservation Program

Physical Sample Preservation and Curation

Geological sample repository holdings typically consist of sediment cores (acquired through drilling and other methods), and other samples types such as rock and mineral specimens, cuttings, peels, fossils, and sediment and soil samples. With the principles of maintaining access and longevity for their holdings in mind, methods for storing geological sample collections are as varied as the types of specimens themselves.

Different types of samples require different storage environments, and methods of pre-storage preparation and curation. As a general rule, all samples are subject to having their fundamental scientific and commercial values increased as knowledge and information about them increase. All are equally subject to lessening of value through physical degradation and loss of information and documentation. 

Issues and Solutions Concerning the Management of Physical Samples

Environmental control

The primary concern for any archival repository is the preservation of integrity for their holdings. Geological sample repositories are no exception. Like analog records, physical sample archives require environmental controls that exclude the ambient environment, and maintain optimal conditions to preserve the longevity of their holdings. Issues such as projected use of the samples and nature of research can dictate the requirements needed to appropriately store these collections.

Ice, Marine and Well Water Samples and Cores

Ice cores at the National Ice Core Laboratory (NICL) are stored in facilities that can maintain temperatures far below freezing (archival temperature is maintained at -35º C) with little fluctuation to avoid accidental melting and the loss of potential chemical and paleoatmospheric information. The cold lab at the NICL facility includes a Class 100 clean room and maintains a temperature of -22º C. Marine and lacustrine soft sediment cores often have refrigerated or freezing storage requirements, as fluctuations in air temperature and humidity can degrade the viability of the samples for geochemical and geophysical properties research, as well as promote organic growth. Preserved water well cores require storage in sealed, air-evacuated tubes containing native groundwater with chemically-enhanced reducing conditions, or are prepared by drying them out under reducing conditions.

Hard Rock Cores and Samples

Rock cores with little to no moisture content need to be protected from damage due to exposure to natural hazards (i.e., extreme weather, etc.). The assurance of reasonably protected indoor storage is widely recognized as crucial for any sample archive. Whether it be in characteristic low-humidity, climate-controlled warehouses such as the USGS Denver Core Research Center (CRC) or the Bureau of Economic Geology (BEG) repository in Houston, or in non-controlled shipping containers in which some new acquisitions are currently held at the Alaska Geological Materials Center.

Unique and Hazardous Samples

Samples that require special and unique methods of preservation are also abundant. Radioactive samples require isolation and either isolation, or systematically controlled ventilation. Samples of gas hydrate require extreme cold (stored in liquid nitrogen dewars) to preserve their integrity, or must be kept under high pressure in Parr vessels.

Sample preparation

Samples collected in the field used for analysis in the lab need to be prepared for the various studies planned to be conducted on them. Preparation of samples prior to incorporation into storage collections varies by sample type, and the nature of study they are to be used for.

Preparing the Sample for Addition to the Collections

Drill core cuttings, typically kept in 50 gram aliquots, are either washed or unwashed, depending on whether drilling mud may be safely removed from the rock type (clay cuttings are generally not washed). Washed and unwashed splits of cuttings may both be retained for archiving. Water well cores may require the environmental control treatment mentioned above (evacuation, reducing treatment, sealed tubes). Ice cores require atmospheric isolation in vapor barrier sleeves inside reduced circulation reflective capped cardboard tubes, beginning at time of removal from core barrel after drilling. For drilled continental rock samples, as well as hand samples and marine rock dredges, the needs for physical care may be less stringent, but need for accessible storage, tracking, and organizational care remain important. At the Kentucky Geological Survey core library, cores are immediately identified and depth intervals, sequence numbers of boxes, total span depth, and condition of core are recorded. Staff members usually retrieve cores from the field, although donors may also deliver to the facility. Containers (boxes) are repaired or replaced as needed, wood crates replaced with cardboard. “Call numbers” are assigned, labels printed, and box numbers (separate from other sample identification numbers) assigned, all before cores are placed on pallets and racked.

Photographic Documentation

Photography is increasingly a standard preparation procedure for newly collected samples. Soft sediment marine and lacustrine cores have begun to be imaged for viewing in high definition and large format, as a primary mode of visual stratigraphy presented over the Web, and through data visualization systems such as Corewall. Ice cores are digitally photographed at high pixel density, and the images intended for internet distribution, to supplant the need for viewing by researchers in a low-temperature environment. At the USGS CRC, photographs of core samples have long been an initial step in sample preparation. Later, plane-polarized light photographs of thin sections from the samples are made and linked to other textual information on the respective samples. Such photographic information can be, and increasingly is, added at any later time to files of other digital-form information about samples: the Kentucky Geological Survey gives specific procedures for adding photographs, drawings, and textual information to the records about samples or suites.

Relocating collections 

Various repositories express differing degrees of caution toward the hazards of moving collections of physical samples. At the Kentucky Geological Survey sample repository, all boxes of samples (including paper records) are assigned sequential numbers, independent of other sample classification codes, for the purpose of re-ordering after moves, and three separate moves of location have been conducted without serious incidents. In contrast, at the Alaska Geological Materials Center moves of portions of the holdings are regarded as perilous undertakings to be avoided, and accommodations and protective structures have been arranged on the long-term main storage land plot in order to avoid the physical disruption of moves of the collection.

Accessing the collections

Sample repositories are obliged to establish policies that balance the security and preservation of collections with the needs of the client communities to extract scientific and economic information from them. These policies highlight issues such as making samples available for destructive analysis, guidance and regulations regarding access to samples for in-lab sample inspection, and lending of samples for non-destructive outside use.

Inside the NICL freezer.At the NICL, samples may have been acquired and may be stored at greater cost than some core samples in other repositories. Sample access, and assistance in preparing subsamples of cores are granted by a sample allocation committee, a subset of the Laboratory’s governing board, on the basis of scientific merit of research proposal, availability and value of ice core segments being requested, and credibility and continuity of the funding of the research investigator making the proposal. This process is conducted by electronic communication, and although procedures are formal, responses to proposing investigators are commonly prompt (often within days).

At many repositories, policies and regulations for acquisition of samples from collections and facilities for in-lab sampling and preparation are prominently and clearly posted on their websites or in other publications. Return of unused portions of samples and estimation of dates of resultant published research documents to be sent to the repositories are typical requirements. The Florida Geological Survey maintains standard research proposal forms on its website, giving a summary of policy requirements and expectations. These include that an assessment of available resources related to the requested samples (e.g., previous analytical results, thin sections) be made prior to the sample approval. And that “value added” products (e.g., thin sections, smear slides) made from the repository samples by the requestors be given to the repository at the completion of the research project.

Shelves in the USGS Core Research CenterThe USGS CRC in Denver has a policy of enhancing the visibility of core characteristics by longitudinally cutting core into half-rounds, thereby saving a large fraction of required storage space by halving the diameter of the archived samples, which are stored in boxes on warehouse shelving. The mating half-rounds of the cores are subsequently made available for redistribution for research. Clients place orders for boxes of core half-rounds to be examined in examination rooms provided in CRC facilities, after CRC employees “pick” the orders from the coded shelved array of boxes, by means of high-lift warehouse vehicles. The CRC, like many other repositories, has specific and typically well-publicized requirements for lending samples for non-destructive purposes.

Some repositories have long standing policies for periodic or scheduled provision of certain types of samples to specific museums with which they have intragovernmental or other formal relationships, as a part of educational outreach programs. These are in some cases loans for specific periods, in others outright grants of samples (e.g., the Florida Geological Survey provides samples to the State Museum of Natural History). 

There is some indication of a future trend toward the development of discrete, semi-external governing bodies to oversee both the curation and distribution of samples, and to advise and oversee the research to be conducted on them. An example of this is a White Paper report, now in preparation, to propose the establishment of a university consortium to undertake this work for the U.S. collection of (polar) ice cores and the climate change, biological, and paleoatmospheric research that is done on them. In the current state of the proposal, a Science Management Office would be intermediate between the consortium board and the ice repository.

Repositories face varying external threats to their collections. At the Kenyan National Anthropological Collections, extensive structural and procedural security precautions have been taken to prevent unauthorized entry and destruction of the collection, reportedly by anti-evolution extremists. The Danish national ice core lab has actively decided to limit public disclosure of the exact physical location of the storage collection, purportedly due to similar concerns. At the U.S. National Ice Core Laboratory, security precautions are at an intermediate level, but staff express concerns about possible un-authorized or non-scientific interest in time-stratigraphic intervals of historical or religious-cultural significance. More broadly, some repository professionals advocate the concept of omitting from publically-accessible records (sample coding) information relating to the specific collection location of samples of sensitive economic or paleontological interest.

Physical aspects of storage

Repositories are typically under substantial logistical and financial restriction, and for those reasons options for construction of fully desirable physical storage facilities are widely constrained. For some repositories, formal plans have been put forth for improvement of aspects of physical plants, and a proposal is current for an extensive construction project for a totally new repository. Styles of shelving, completeness of environmental (temperature and humidity) control, boxing for core, and types of containers for non core samples vary widely. The strong discrepancy between high costs of sample acquisition and relatively low costs of sample storage typically lead repositories to have policies to retain holdings even if they are not in accessible storage, and for that reason holdings at even the best repositories may be seen in obsolete wooden core boxes, reclaimed glass containers, non-environmentally controlled buildings, and ocean shipping containers that are difficult of access. Repositories state that better storage environments (climate control, space-efficient shelving, uniformity of containers, etc.) are a long term goal, while at present concentrating on efforts to retain valuable present and increasing holdings of samples. The websites and other published documents of many or most repositories give candid descriptions of their physical plants and physical methods of sample storage. See further discussion below, in section on facility design, infrastructure, storage, location. 

How can you contribute?

If you have any success stories, suggestions, or contributions relating to physical sample preservation you would like to share, please send an email to the USGS National Geological and Geophysical Data Preservation Program at nggdpp@usgs.gov.  We will work with you to highlight your best practices and share them online.