The Interpretation of Biogeochemical Growths on Gold Coins from the SS Central America Shipwreck: Applications for Biogeochemistry and Geoarchaeology

Journal of Marine Science and Engineering

Article The Interpretation of Biogeochemical Growths on Gold Coins from the SS Central America Shipwreck: Applications for Biogeochemistry and Geoarchaeology

Erik B. Melchiorre 1,2,* , Bryan H. Seymour 2 and Robert D. Evans 3

1 Department of Environmental Sciences, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA 2 California State University, San Bernardino, Department of Geological Sciences 5500 University Parkway, San Bernardino, CA 92407, USA 3 American Numismatic Association, Colorado Springs, CO 80903, USA * Correspondence: [email protected]; Tel.: +01-909-537-7754

Received: 29 May 2019; Accepted: 27 June 2019; Published: 5 July 2019

Abstract: Black crusts that formed on gold coins recovered from the 1857 shipwreck of the SS Central America played a key role in their preservation in a near original state. Within a few years of the sinking, the signiﬁcant quantities of iron and steel on the shipwreck produced laminar geochemical precipitates of ﬁne-grained iron minerals on the coins. This coating served to armor the coins from future chemical or biological attacks. Once coated, the coins were colonized by at least two distinct populations of gold-tolerant bacteria that precipitated abundant nanoparticulate gold in the black crust material and produced biomineralized bacteria in a web-like mat. Above this middle layer of black crust, the outer layer consisted of a geochemical reaction front of euhedral crystals of iron sulfate and iron oxy-hydroxide species, formed by the interaction of seawater with the chemical wastes of the bacterial mat. Understanding this process has application for assessing the diverse and extreme conditions under which nano-particulate gold may form through biological processes, as well as understanding the conditions that contribute to the preservation or degradation of marine archaeological materials.

Keywords: gold; nanoparticulate; C. metallidurans; SS Central America; biomineralization

1. Introduction

1.1. Significance of the California Gold Rush The decade following the discovery of gold in California was arguably one of the most catalytic times in US history. The California Gold Rush greatly accelerated western migration by Europeans, and eastern migration by Asians and Pacific Islanders. Furthermore, it significantly strengthened the economy of the young United States of America, and stripped native peoples of their ancestral lands, killing up to 80% of their population [1–3]. In just two short years, California went from a remote and sparsely populated outpost of Mexico to US statehood. James Marshall is credited with the official discovery of gold in California on January 24, 1848 at the site of a sawmill he was building for John Sutter on the American River [4], though gold was known to occur in California far earlier [5,6]. Yet, little remains of the original geological materials, coins, and other artifacts from the first decade of this crucial period of American history. For example, period coins can provide insights on commerce patterns through wear patterns if verifiably preserved from the early days of the gold rush. Hoards or

J. Mar. Sci. Eng. 2019, 7, 209; doi:10.3390/jmse7070209 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 209 2 of 16 caches of period coins may also reveal the existence or prevalence of contemporary counterfeiting. Similarly, preserved placer gold samples can provide insights on which areas were mined, when they were mined, and even what techniques were used. However, the vast majority of placer gold and gold coins from this period have not survived the melting pot, having long ago been converted into new articles. Furthermore, surviving specimens have limited geochemical or archeological context, having often been subjected to aggressive cleaning, subsequent wear, or both. There is also the issue of being certain of an item’s provenance or historical authenticity. The scarcity of such direct archeological and geological evidence from the gold rush period of California was signiﬁcantly lessened with the discovery of the SS Central America shipwreck site, providing scientists with a trove of material as a well-preserved time-capsule from 1857.

1.2. The Loss of the SS Central America The SS Central America was a steam-powered side-wheel ship, 85 m (278 feet) long and 12 m (40 feet) wide [7]. The voyage for the passengers and materials of the SS Central America began when the SS Sonora commenced the two-week west coast leg of the journey, departing from San Francisco on 20 August, 1857 and bound for Panama (Figure1). From Panama, the passengers and cargo from the SS Sonora, and several smaller ships, crossed the isthmus by railroad and re-embarked on the SS Central America on September 8, 1857 for a nine-day journey to New York [4]. Aboard the ship was a broadly reported commercial shipment of $1,219,189.43 in gold. Pure gold at the time carried a value of $20.672 per troy ounce (31.1035 g). Approximately 87% of the gold’s face value in the commercial shipment consisted of assayers’ ingots, essentially the raw placer gold or other gold (like foreign coins) melted and poured into bar form with the precise weight and purity determined during the process. These gold bricks, as well as the placer gold found on the site, consisted on average of ~88% gold with the remainder mostly silver. The US federal gold coins, that mostly comprise the other 13% of the commercial shipment, are made of an alloy of 90% gold and 10% copper, sometimes with small amounts of silver substituting for copper in the mix. Some contemporary accounts estimate that the passengers may have carried a cumulative amount equal to this 2,000-plus kilogram commercial shipment. The first three days at sea, the SS Central America battled rough weather, but with a seasoned captain and crew, and a large, powerful steam engine to instill confidence, the conditions were not deemed hazardous enough to return to port. What captain William Lewis Herndon and his crew had no way of knowing, was that they were sailing on an intercept course with a category two hurricane [8]. On 11 September, 1857, under worsening seas, a leak at one of the side-wheel paddle gaskets began filling the engine room with sea water, shutting down the steam engines and bilge pumps, and sealing the fate of the ship [7]. All the female passengers and all but one of the children were rescued by small ships in the area, while the men fought to save the sinking ship and its fortune of gold. The ship sank the next day at 8:00 pm, claiming the lives of 425 passengers and crew, and a reasonably approximated 4000 kilograms of gold. The news of the loss of the significant, documented, commercial gold shipment (modern gold value of approximately US$75 million) plus an unknown amount of passenger wealth deepened the Financial Panic of 1857, which crippled the nation up through the start of the American civil war [9].

1.3. Discovery of the Shipwreck, and Recovery Efforts In 1983, Tommy Thompson, an ocean engineer, conceived a project to search for the shipwreck site of the SS Central America scientifically. Utilizing historical information in a complex mathematical model using Bayesian search theory and the critical path method, his company generated a search probability map to aid in finding the sunken ship. Historical data used in the model included wind speeds during the storm, the condition of the vessel, known locations from the ship and rescue ships, and accounts of the sinking provided by the survivors [7]. Review of sonar records within the search areas revealed an anomaly that the experts initially deemed a “large geological feature.” J. Mar. Sci. Eng. 2019, 7, 209 3 of 16 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 3 of 17

Figure 1. Map of the route of the SS Sonora from San Francisco (1), to Panama (2), where passengers embarked on the SS Central America forfor Cuba Cuba (3) (3) with an ultimate destination of of New New York. York. The The ship ship sank in the the Atlantic Atlantic Ocean Ocean on on September September 12, 12, 1857 1857 duri duringng a hurricane hurricane off oﬀ the coast of the Carolinas Carolinas (4).

1.3. DiscoveryLocated 150of the nautical Shipwr mileseck, and (280 Recovery km) off theEfforts Carolina coast on a transitional feature of the continental slope, known as the Blake Ridge, this site was first explored using the NEMO-ROV submersible on In 1983, Tommy Thompson, an ocean engineer, conceived a project to search for the shipwreck the anniversary week of the sinking, on 11 September, 1988. Upon reaching the seafloor, there was site of the SS Central America scientifically. Utilizing historical information in a complex no doubt that this was the shipwreck of the SS Central America. The large mid-ship paddle wheel mathematical model using Bayesian search theory and the critical path method, his company and other distinctive features were the first signs. However, confirmation was provided when they generated a search probability map to aid in finding the sunken ship. Historical data used in the observed that the seafloor was littered with gold, in an area that became known as the “garden of gold” model included wind speeds during the storm, the condition of the vessel, known locations from the (Figure2). The excavation that followed took four summer seasons to recover over 1,900 kg (two tons) ship and rescue ships, and accounts of the sinking provided by the survivors [7]. Review of sonar of theJ. lostMar. Sci. gold. Eng. 2019, 7, x FOR PEER REVIEW 4 of 17 records within the search areas revealed an anomaly that the experts initially deemed a “large geological feature.” Located 150 nautical miles (280 km) off the Carolina coast on a transitional feature of the continental slope, known as the Blake Ridge, this site was first explored using the NEMO-ROV submersible on the anniversary week of the sinking, on 11 September, 1988. Upon reaching the seafloor, there was no doubt that this was the shipwreck of the SS Central America. The large mid-ship paddle wheel and other distinctive features were the first signs. However, confirmation was provided when they observed that the seafloor was littered with gold, in an area that became known as the “garden of gold” (Figure 2). The excavation that followed took four summer seasons to recover over 1,900 kg (two tons) of the lost gold.

FigureFigure 2. Seaﬂoor 2. Seafloor image image of of gold gold from from the the shipwreck shipwreck of the the SS SS CentralCentral America America. Note. Note the theformation formation of of blackblack crusts crusts on the on coinsthe coins and and some some of theof the gold gold bars. bars These. These crusts crusts appear appearblack black toto greygrey in the image, due to constraintsdue to constraints of seaﬂoor of seafloor lighting. lighting. Image Image courtesy courtesy of California of California Gold Gold Marketing Marketing Group. Group.

In April of 2014, a second recovery team went to sea for five months, retrieving an additional 3,100 gold coins including hundreds recovered from within an iron safe. Furthermore, inside the safe were bags, vests, and saddle bags containing placer gold dust and gold-quartz specimens. Just as with the earlier recovery, the surfaces of these gold coins (but rarely the placer gold) were coated with black crusts. These crusts on the artifacts are painstakingly removed with mild solutions that do not disturb or react with the underlying gold (Figure 3). However, the questions remain: what are these black crusts, and why do they form on a non-reactive noble metal?

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In April of 2014, a second recovery team went to sea for ﬁve months, retrieving an additional 3100 gold coins including hundreds recovered from within an iron safe. Furthermore, inside the safe were bags, vests, and saddle bags containing placer gold dust and gold-quartz specimens. Just as with the earlier recovery, the surfaces of these gold coins (but rarely the placer gold) were coated with black crusts. These crusts on the artifacts are painstakingly removed with mild solutions that do not disturb or react with the underlying gold (Figure3). However, the questions remain: what are these black crusts,J. Mar. Sci. and Eng. why 2019 do, 7, theyx FOR form PEER onREVIEW a non-reactive noble metal? 5 of 17

FigureFigure 3. 3.Image Image ofof somesome ofof thethe goldgold recoveredrecovered from the shipwreck shipwreck of of the the SS SS CentralCentral America America afterafter curatorialcuratorial treatment. treatment. The The black black crusts crusts have have been been retained retained on on some some assay assay bars bars (upper (upper right right and and left) left) for estheticfor esthetic purposes. purposes. Image Image courtesy courtesy of California of California Gold Gold Marketing Marketing Group. Group.

1.4.1.4. Previous Previous Work Work TheThe S.S. S.S.Central Central America Americashipwreck shipwreck site site offers offers a myriad a myriad of opportunities of opportunities for scientific for scientific and historical and studies.historical Scientific studies. publications Scientific publications on aspects of on the aspects shipwreck of the began shipwreck with a began 200-plus with page a 200-plus monograph page [7 ], whichmonograph was followed [7], which by was many followed articles, by theses, many articl and dissertations,es, theses, and includingdissertations, works including on the works biology on of thethe site biology (many of the species site (many new to species science), new as to well science) as material, as well as science material and science costume andhistory costume related history to clothingrelated (textiles),to clothing and (textiles), other artifacts and other found artifacts in passenger found in trunks. passenger No previoustrunks. No scientific previous studies scientific have focusedstudies specificallyhave focused on specifically the mineral on crusts the mineral found growing crusts found on the growing SS Central on Americathe SS Centralgold. However,America significantgold. However, studies significant have been studies done on have black been crusts done growing on black on crusts natural growing placer goldon natural [10,11]. placer These gold black crusts[10,11]. on These placer black gold arecrusts associated on placer with gold the are secondary associated formation with the of naturalsecondary placer formation gold particles of natural with ironplacer oxyhydroxide gold particles coatings with iron (young oxyhydroxide biogenic gold)coatings and (young are derived biogenic from gold) anoxic and wetare derived environments, from whichanoxic favor wet microbialenvironments, growth which within favor the microbial microscopic growth cracks within of the placer microscopic gold [12– cracks14]. In of addition, placer deep-seagold [12–14]. corrosion In addition, studies deep-sea [15] provide corrosion valuable studie informations [15] provide on sources valuable of information reactants for on the sources formation of ofreactants secondary for crusts the formation such as thoseof secondary found on crusts the gold such from as those the SSfoundCentral on the America gold. from the SS Central America.

J. Mar. Sci. Eng. 2019, 7, 209 5 of 16

2. Materials and Methods Five samples of black crusts were obtained from the surface of $20 (one troy ounce) gold coins that were recovered from the SS Central America shipwreck site. Each sample weighed ~0.1 gram and consisted of small flakes of material up to 1 cm on the longest axis. Removal of these crusts was facilitated by soaking in a sodium salt solution. This exact methodology and chemistry is proprietary, and was performed by Robert Evans at laboratory facilities in southern California. These samples were providedJ. Mar. courtesySci. Eng. 2019 of, 7, California x FOR PEER REVIEW Gold Marketing Group, the owner of the treasure. 6 of 17 Black crust samples were structurally and chemically characterized, using methods described by [10These], to determinesamples were possible provided biogenicity. courtesy of Samples California were Gold placed Marketing on their Group, flattest the sideowner to exposeof the the maximumtreasure. surface area and photographed under a binocular microscope. A Phenom XL Scanning Electron MicroscopeBlack crust samples (SEM) was were used structurally to capture and Secondary chemically Electron characterized, (SE) and using Backscatter methods described Electron (BSE) imagesby using[10], to an determine acceleration possible voltage biogenicity. of 15 kV. Samples Samples were were placed not on carbon their flattest coated, side but to wereexpose grounded the maximum surface area and photographed under a binocular microscope. A Phenom XL Scanning using carbon tape. Porous zones and zones of variation in surface texture were targeted for analysis. Electron Microscope (SEM) was used to capture Secondary Electron (SE) and Backscatter Electron The same(BSE) instrumentimages using was an usedacceleration for Energy voltage Dispersive of 15 kV. X-raySamples (EDX) were element not carbon measurements coated, but were of major and minorgrounded element using abundances,carbon tape. Porous using zones standard and ozonesff-peak of variation interference in surface and matrix texture corrections were targeted [16 ,17], in additionfor analysis. to calibrations The same usinginstrument registered was standardsused for Energy provided Dispersive by the manufacturer.X-ray (EDX) element The error associatedmeasurements with the of in-situ major traceand minor element element analyses abunda is lessnces, than using 2%.standard off-peak interference and ± matrix corrections [16,17], in addition to calibrations using registered standards provided by the 3. Resultsmanufacturer. The error associated with the in-situ trace element analyses is less than ±2%. The $20 gold coins from which the black crusts were recovered are dated 1857, with a San Francisco 3. Results mint mark. These coins are Liberty Head double eagles, a design by James Longacre, which was in productionThe between $20 gold 1850 coins and from 1907 which [18]. the These black coins crusts are were in uncirculated recovered are condition dated 1857, and presumedwith a San to be Francisco mint mark. These coins are Liberty Head double eagles, a design by James Longacre, part of the large shipment of these coins that was being transported by businesses to the east coast of which was in production between 1850 and 1907 [18]. These coins are in uncirculated condition and the country. A representative sample of one of these coins, superimposed with an image of a fragment presumed to be part of the large shipment of these coins that was being transported by businesses to of black crust is provided in Figure4 to show relative positioning.

FigureFigure 4. Image 4. Image of an of 1857-San an 1857-San Francisco Francisco mint mint $20 gold $20 coingold fromcoin thefrom shipwreck, the shipwreck, showing showing the preserved the cast ofpreserved the coin cast on aof fragment the coin ofon blacka fragment crust (redof blac box).k crust The (red image box). of The the crustimage hasof the been crust inverted has been to align with theinverted coin, to as align it is awith negative-relief the coin, as cast.it is a The negati yellowve-relief areas cast. on The the crustyellow are areas not on gold, the butcrust hydrated are not iron oxides.gold, Coin but imagehydrated courtesy iron oxides. of Professional Coin image Coincourtesy Grading of Professional Service. Coin Grading Service.

TheThe black black crusts crusts are are<1 <1 mm mm thick thick and and fragile.fragile. The The outer outer surfaces surfaces have have a porous a porous dull dullblack black to to dark-reddark-red color. color. The surfacesThe surfaces that werethat were in contact in contact with with the coin the formcoin form a negative-relief a negative-relief cast thatcast faithfullythat faithfully reproduces details of the coin (Figure 4), and are vitreous, laminated, and smooth. reproduces details of the coin (Figure4), and are vitreous, laminated, and smooth. The morphology of all black crust samples examined under the binocular and SEM microscopes were found to consist of an identical three layers (Figure 5A). The lower-most layer, deposited directly on the surface of the coin, exposes a smooth uniform surface (Figure 5B). The middle layer has “spongy” texture of elongate mineralized structures and cell-like forms with abundant nano-particulate gold (Figure 5C). The upper-most layer consists of blocky well-formed euhedral crystals up to 25 microns in length (Figure 5D). J. Mar. Sci. Eng. 2019, 7, 209 6 of 16

The morphology of all black crust samples examined under the binocular and SEM microscopes were found to consist of an identical three layers (Figure5A). The lower-most layer, deposited directly on the surface of the coin, exposes a smooth uniform surface (Figure5B). The middle layer has “spongy” texture of elongate mineralized structures and cell-like forms with abundant nano-particulate gold (Figure5C). The upper-most layer consists of blocky well-formed euhedral crystals up to 25 microns in length (FigureJ. Mar. Sci.5 Eng.D). 2019, 7, x FOR PEER REVIEW 7 of 17

Figure 5.FigureSEM 5. imagesSEM images of the of threethe three distinct distinct layers layers withinwithin black black crusts crusts at 1,100×: at 1,100 coin: surface, coin surface, biomat biomat × layer, andlayer, outer and layer.outer layer. (A) Exposed (A) Exposed section section of theof the three three layers layers along along aa naturalnatural fracture, fracture, (B ()B layer) layer 1 1 with letter imprintwith letter from imprint coin from surface coin (630 surface) and (630×) crypto-crystalline and crypto-crystalline iron iron oxides oxides (grey), (grey), ( C(C)) middle middle layer 2 layer 2 with spongy growths of iron× oxides (grey), and (D) surface layer 3 with euhedral iron sulfate with spongy growths of iron oxides (grey), and (D) surface layer 3 with euhedral iron sulfate minerals minerals (dark grey) (C and D; both 5,900×). (dark grey) (C and D; both 5,900 ). × The lowermost black crusts consist of smooth weakly laminated layers of iron sulfate and less Thecommonly lowermost iron blackoxide crustsminerals, consist as determined of smooth by EDX. weakly It was laminated not possible layers to determine of iron sulfatethe exact and less commonlyspeciation iron oxideof these minerals, minerals by as X-ray determined diffraction by due EDX. to the It small was quantity not possible of samples to determine and access to the exact speciationinstrumentation. of these minerals Desiccation by X-ray cracks di areffraction common due on to this the layer. small Crystals quantity >0.1 of micron samples are andrarely access to instrumentation.observed in Desiccation association with cracks this are layer. common This layer on thisfaithfully layer. reproduces Crystals >a0.1 negative micron cast are of rarelythe coin observed substrate to which it was attached, with preservation of details down to the micron scale. in association with this layer. This layer faithfully reproduces a negative cast of the coin substrate to The middle crust consists of iron oxide (hydroxide) minerals, siderite (<2% Ca), calcium sulfate which it(hydroxide) was attached, minerals, with preservationorganic debris offrom details the downshipwreck, to the base micron metal scale. sulfides (e.g., pyrite, Thechalcopyrite, middle crust and consists galena), of and iron abundant oxide (hydroxide) nano-particulate minerals, gold. siderite Attached (< 2%to Ca),the calciumelongate sulfate (hydroxide)pseudo-crystals minerals, of organic the iron debris oxide/iron from sulfate the shipwreck, minerals are base abundant metal sulfidesnano-particulate (e.g., pyrite, gold grains, chalcopyrite, and galena),typically and one abundant micron or nano-particulate less in size with gold. an alloy Attached chemistry to thedescribed elongate below. pseudo-crystals There are many of the iron oxide/ironpotential sulfate hydrated minerals iron are oxide abundant mineral nano-particulate species, and 14 known gold grains,hydrated typically calcium sulfate one micron mineral or less in species. However, the exact speciation of the iron oxides and iron sulfates is unknown due to size withinstrument an alloy access chemistry and sample described quantities, below. which There were are insufficient many potential for proper hydrated XRD characterization. iron oxide mineral species, andThe 14 knownuppermost hydrated crust layer calcium consists sulfate of several mineral generations species. of iron However, oxides, siderite the exact (<4% speciation Ca), iron of the iron oxidessulfate and minerals, iron sulfates and rarely is unknownpyrite, chalcopyrite, due to instrument and galena. accessThese crusts and samplecontain some quantities, combination which were insufficientof the for 19 properknown hydrated XRD characterization. iron oxide species and 36 known hydrated iron sulfate species. However, Theas uppermostmentioned above, crust the layer determination consists of of several exact sp generationseciation was beyond of iron the oxides, scope sideritepossible with (<4% the Ca), iron existing study and instrumentation. Furthermore, the exact mineralogy of the original hydrated sulfate minerals, and rarely pyrite, chalcopyrite, and galena. These crusts contain some combination of crusts on the deep seafloor has probably been altered to some degree by the recovery and drying of the samples. If this has indeed occurred, it is likely that the well-defined crystal forms in this layer

J. Mar. Sci. Eng. 2019, 7, 209 7 of 16 the 19 known hydrated iron oxide species and 36 known hydrated iron sulfate species. However, as mentioned above, the determination of exact speciation was beyond the scope possible with the existing study and instrumentation. Furthermore, the exact mineralogy of the original hydrated crusts on the deep seafloor has probably been altered to some degree by the recovery and drying of the samples. If this has indeed occurred, it is likely that the well-defined crystal forms in this layer are pseudomorphs that preserveJ. Mar. Sci. the Eng. form 2019, of7, x the FORoriginal PEER REVIEW seafloor species. This in turn complicates the potential 8 offor 17 existing mineral identification based on crystal structure. Future expeditions to the shipwreck site may produce fresh materialare pseudomorphs that can be that properly preserve preserved the form of in the cold orig seawater,inal seafloor in quantitiesspecies. This that in turn will complicates permit a detailed mineralogicalthe potential study for of existing these crusts.mineral identification based on crystal structure. Future expeditions to the shipwreck site may produce fresh material that can be properly preserved in cold seawater, in Somequantities elements that will such permit as elementala detailed mineralogical sulfur and study some of these metals crusts. such as molybdenum are found irregularly distributedSome elements through such as onlyelemental the middlesulfur and crust some layer, metals and such in closeas molybdenum association are with found potential biogenicirregularly forms that distributed are similar through in size only and the shapemiddle tocrus cellt layer, structures and in close (Figure association6A,B). Otherwith potential features such as microfossilsbiogenic (5–100forms that microns) are similar and in organicsize and shape debris to fromcell structures the shipwreck (Figure 6A,B). were Other found features embedded such in the outermostas microfossils two layers (5–100 of the microns) black crusts and organic (Figure debris6C,D). from Sodium the shipwreck carbonate were andfound sodium embedded sulfate in the crystals outermost two layers of the black crusts (Figure 6C,D). Sodium carbonate and sodium sulfate form overgrowths on some outer portions of the crusts (Figure6E). Quartz sand grains with conchoidal crystals form overgrowths on some outer portions of the crusts (Figure 6E). Quartz sand grains with fracturesconchoidal are also fractures found within are also the found crusts within (Figure the crusts6F). (Figure 6F).

Figure 6.FigureSEM 6. images SEM images of features of features from from black black crusts. crusts. (A ,(BA),B Cell-like) Cell-like forms forms fromfrom thethe middle crust, crust, (C,D) remains(C of,D microscopic) remains of microscopic marine life marine embedded life embedded within within black crustsblack crusts (highlighted (highlighted by by arrows), arrows), ( E(E)) calcium sulfate crystalscalcium sulfate (grey) crystals with sodium (grey) with carbonate sodium carbonate overgrowths overgrowths (white), (white), and ( Fand) a ( conchoidallyF) a conchoidally fractured fractured sand grain (center of image). sand grain (center of image). Nanoparticulate gold is present throughout the uppermost two layers, but is particularly abundant in the middle layer. These gold grains vary in size from 0.2 to 12 microns in length along the longest axis and show a wide range of alloy chemistry (Figure 7). Nanoparticulate gold grains

J. Mar. Sci. Eng. 2019, 7, 209 8 of 16

Nanoparticulate gold is present throughout the uppermost two layers, but is particularly abundant inJ. Mar. the Sci. middle Eng. 2019 layer., 7, xThese FOR PEER gold REVIEW grains vary in size from 0.2 to 12 microns in length along the longest 9 of 17 axis and show a wide range of alloy chemistry (Figure7). Nanoparticulate gold grains ranged from ranged from 85.5 to 2.1 wt.% gold, with an average of 58.9 wt.% gold. Silver ranged from 28.5 to 85.5 to 2.1 wt.% gold, with an average of 58.9 wt.% gold. Silver ranged from 28.5 to <0.01 wt.%, with <0.01 wt.%, with an average of 17.1 wt.%, but was not present in all samples. Copper in the gold an average of 17.1 wt.%, but was not present in all samples. Copper in the gold alloy ranged from 10.9 alloy ranged from 10.9 to <0.01 wt.%, with an average of 4.7 wt.%, but similarly was not present in to <0.01 wt.%, with an average of 4.7 wt.%, but similarly was not present in measurable quantities in measurable quantities in all samples. Iron was present in significant quantities in all but one sample all samples. Iron was present in signiﬁcant quantities in all but one sample of nanoparticulate gold, of nanoparticulate gold, and ranged from 93.6 to 7.5 wt.%, with an average of 25.9 wt.%. Sulfur, and ranged from 93.6 to 7.5 wt.%, with an average of 25.9 wt.%. Sulfur, when present, ranged from 21.0 when present, ranged from 21.0 to 4.3 wt.%, with an average of 13.5 wt.%. Loose material from a to 4.3 wt.%, with an average of 13.5 wt.%. Loose material from a crust with high iron nanoparticuate crust with high iron nanoparticuate gold was sonicated and re-analyzed to reveal an iron content gold was sonicated and re-analyzed to reveal an iron content below detection limits of 0.01 wt.%. below detection limits of 0.01 wt.%.

FigureFigure 7.7. TernaryTernary plotplot ofof ﬁfty-sixfifty-six chemicalchemical analysesanalyses ofof nanoparticulatenanoparticulate goldgold alloysalloys fromfrom blackblack crustscrusts (blue(blue dots).dots). NoteNote thethe exaggeration exaggeration of of the the Cu Cu axis axis used used to to magnify magnify variations variations of of this this element element relative relative to theto the other other two. two. The The composition composition of an of 1857-San 1857-S US $20US gold$20 gold coin coin (yellow (yellow star), star), average average placer placer gold gold (red diamond),(red diamond), and assayand assay ingots ingots (purple (purple square) square) from from the SS theCentral SS Central America Americaare shown are shown for reference. for reference.

4.4. Discussion 4.1. Mineral Crust Formation 4.1. Mineral Crust Formation The smooth and finely laminated black crust layer that was in direct contact with the coins The smooth and finely laminated black crust layer that was in direct contact with the coins contains well-preserved casts of letters and other details from the coins (Figures4 and5B), in addition contains well-preserved casts of letters and other details from the coins (Figures 4 and 5B), in to very fine-grained mineralization. These data suggest that this layer was deposited by relatively addition to very fine-grained mineralization. These data suggest that this layer was deposited by rapid precipitation from a supersaturated source that provided many fine-grained nucleation points. relatively rapid precipitation from a supersaturated source that provided many fine-grained The Fe-dominated chemistry of this layer likely reflects the initial dissolution of the 750 tons of ironwork nucleation points. The Fe-dominated chemistry of this layer likely reflects the initial dissolution of the 750 tons of ironwork found within the ship, and its re-precipitation as stable minerals through reactions with seawater. Desiccation cracks in this layer are probably the result of drying following recovery of the coins and their attached black crusts from the shipwreck.

J. Mar. Sci. Eng. 2019, 7, 209 9 of 16 found within the ship, and its re-precipitation as stable minerals through reactions with seawater. Desiccation cracks in this layer are probably the result of drying following recovery of the coins and their attached blackJ. Mar. crusts Sci. Eng. from 2019, the 7, x shipwreck.FOR PEER REVIEW 10 of 17 The middle “biomat” layer contains significant amounts of nano-particulate gold alongside mineralized cell-likeThe material, middle biological “biomat” debris,layer contains and other significant mineral amounts growths. of Thenano-particulate diverse mineral gold alongside species occurringmineralized as micron-scale cell-like euhedral material, crystalsbiological within debris the, and middle other crustmineral (e.g., growths. galena, The pyrite, diverse mineral and chalcopyrite)species indicate occurring that the coins’as micron-scale environment euhedral changed crystals significantly within the as middle the shipwreck crust (e.g., continued galena, pyrite, and to degrade and reactchalcopyrite) with seawater. indicate The that significance the coins’ environment of this layer ofchanged black crust significantly is described as the in shipwreck detail in continued to degrade and react with seawater. The significance of this layer of black crust is described in detail Section 4.2. in Section 4.2. The outer layer consists of a network of euhedral crystal intergrowths of iron oxide and iron The outer layer consists of a network of euhedral crystal intergrowths of iron oxide and iron sulfate minerals with rare isolated nano-particulate gold grains. The dominance of iron minerals sulfate minerals with rare isolated nano-particulate gold grains. The dominance of iron minerals as as larger well-formed crystals is most likely due to their formation at a time when corrosion of the larger well-formed crystals is most likely due to their formation at a time when corrosion of the shipwreck iron hadshipwreck reached iron a lower had steady reached state a andlower provided steady minimalstate and saturation, provided and minimal the interaction saturation, and the of these fluids withinteraction the metabolic of these products fluids with from the the metabolic biomat layerproducts beneath from it.the The biomat low abundancelayer beneath of it. The low transition-groupabundance sulfide minerals of transition-group such as galena sulfide and chalcopyrite minerals such within as galena this outer and crustchalcopyrite suggests within that this outer these elements werecrust locally suggests depleted that these from elements the shipwreck, were locally or armored depleted from from further the corrosionshipwreck, by or their armored from own corrosion productsfurther corrosion at this time. by their Sodium own carbonate corrosion andproducts sodium at this sulfate time. crystals Sodium form carbonate overgrowths and sodium sulfate on some portionscrystals of the outerform overgrowths crusts (Figure on6 E)some and portions are likely of producedthe outer crusts by the (Figure drying 6E) of theand sodium are likely produced salt solutions usedby tothe clean drying the of black the sodium crusts from salt solutions the coin surface,used to clean and evaporating the black crusts seawater. from the coin surface, and evaporating seawater. 4.2. Biomats and Nanoparticulate Gold Black crusts4.2. on placerBiomats gold and areNanoparticulate believed to formGold due to iron and manganese depositing bacteria [14, 19]. The SS Central AmericaBlack crustsshipwreck, on placer with gold several are believed hundred to tonsform of due cast to iron iron andand carbonmanganese steel depositing steam bacteria plant and ballast,[14,19]. provides The an SS ideal Central location America for shipwreck, similar biogenic with several deposits hundred to form tons in relativelyof cast iron anoxic and carbon steel locations in the seafloorsteam plant environment and ballast, [20 provides]. There an are ideal two componentslocation for similar to the biogenic middle layer deposits of the to form black in relatively crusts from the goldanoxic coins locations from thein the SS seafloorCentral Americaenvironment. The [20]. first There is the are purported two components biomat layer to the itself, middle layer of while the secondthe is theblack nanoparticulate crusts from the gold gold contained coins from within the SS the Central biomat America layer.. The first is the purported biomat layer itself, while the second is the nanoparticulate gold contained within the biomat layer. 4.2.1. Biomats 4.2.1. Biomats Upon examination, the middle layer of the black crusts is distinctly different from the layers above and below it (FigureUpon examination,6A,B and Figure the middle8). In layer addition of the to black fossils cr ofusts various is distinctly microscopic different sea from the layers life forms (Figureabove6C,D), and there below is abundantit (Figure 6A,B evidence and Figure of possible 8). In addition bacterial to fossils. fossils of The various mineralized microscopic sea life morphologic formsforms that (Figure are present 6C,D), have there virtually is abundant identical evidence scale and of shape possible to published bacterial images fossils. of The both mineralized morphologic forms that are present have virtually identical scale and shape to published images of live bacteria that are known to grow on metal substrates (Figure8A,D; [ 21,22]), and biomineralized both live bacteria that are known to grow on metal substrates (Figure 8A,D; [21,22]), and bacteria (Figure8B,E; [23,24]). biomineralized bacteria (Figure 8B,E; [23,24]).

Figure 8. Cont.

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FigureFigure 8. 8.Similarities Similarities of of known known biofilms biofilms compared compared to to the the biofilms biofilms found found within within the the middle middle layer layer of of the blackthe crusts,black crusts, as viewed as viewed using SEM.using (ASEM.) Staphylococcus (A) Staphylococcus aureus aureusbacteria, bacteria, part of part the firmicutesof the firmicutes phylum, easilyphylum, identified easily by identified their ovoid by shapetheir ovoid and conjoinedshape and behavior, conjoined averaging behavior, two averaging microns two in diametermicrons in [22 ]. (B)diameter Image of [22]. potential (B) ImageS. aureus of potentialbacteria S. biomataureus bacteria from the biomat middle from layer the of middle gold coin layer black of gold crust coin from theblack SS Central crust Americafrom theshipwreck. SS Central (AmericaC) Biofilm shipwreck. (sulphate-reducing) (C) Biofilm (sulphate-reducing) grown under anaerobic grown conditions under anaerobic conditions on the surface of mild carbon steel [21]. These bacteria are elongate, averaging on the surface of mild carbon steel [21]. These bacteria are elongate, averaging ten microns in length, ten microns in length, and arcuate, creating a dendritic colony form. (D) Heavily biomineralized and arcuate, creating a dendritic colony form. (D) Heavily biomineralized filamentous bacteria colony filamentous bacteria colony (C. metalidurans) growing on the surface of a placer gold nugget [23]. (E) (C. metalidurans) growing on the surface of a placer gold nugget [23]. (E) Probable mineralized biofilm Probable mineralized biofilm from the middle layer of a gold coin black crust from the SS Central from the middle layer of a gold coin black crust from the SS Central America shipwreck. Sub-micron America shipwreck. Sub-micron nano-particulate gold (white dots) is present throughout these nano-particulate gold (white dots) is present throughout these biofilms. biofilms. It should be noted that nonbiological structures like gas bubbles, spherical abiotic mineral It should be noted that nonbiological structures like gas bubbles, spherical abiotic mineral growths, and amorphous masses (bacteriomorphs) may have characteristics of coccoid bacteria [25]. growths, and amorphous masses (bacteriomorphs) may have characteristics of coccoid bacteria [25]. However, rod-shaped bacteria are more readily differentiated from abiotic growths [25]. It is likely However, rod-shaped bacteria are more readily differentiated from abiotic growths [25]. It is likely thatthat the the rod rod/filamentous/filamentous and and ovoid ovoid shapedshaped growthsgrowths from the the middle middle layer layer of of the the black black crusts crusts from from thethe SS SSCentral Central America Americaare are mineral mineral replacement replacement oror permineralizationpermineralization of of bacteria. bacteria. This This assertion assertion is is basedbased on on the the standards standards presented presented inin [25[25],], includingincluding morphological characteristics, characteristics, formation formation in ina a favorablefavorable environment, environment, and and environmental environmental factorsfactors which are are not not favorable favorable for for the the formation formation of ofthe the mostmost common common bacteriomorphs. bacteriomorphs. ThereThere are are two two putative putative bacterial bacterial morphologies morphologies that that dominate dominate the the middle middle black black crust crust layer. layer. The The first is similarfirst is similar to Staphylococcus to Staphylococcus aureus aureusbacteria bacteria (Figure (Figure8A; [ 21 8A;]), with[21]), clusterswith clusters of conjoined of conjoined ovoid ovoid shapes averagingshapes averaging two microns two inmicrons diameter. in diameter. Biomineralized BiomineralizedS. aureus S. aureusbacteria bacteria from afrom modern a modern biofilm biofilm grown ongrown stainless on steelstainless displays steel nearlydisplays identical nearly identical characteristics characteristics [24]. The [24]. individual The individual ovoids ovoids of the putativeof the mineralizedputative mineralizedS. aureus bacteria S. aureus colonial bacteria clusters colonial from clusters the middle from layerthe middle of gold layer coin blackof gold crusts coin fromblack the SScrustsCentral from America the SShave Central a “deflated” America have look a “deflated” that is diff lookerent that from is different the live fromS. aureus the livecells. S. aureus Thisis cells. likely dueThis to theis likely collapse due of to these the fragilecollapse mineralized of these fragile structures mineralized during structures recovery ofduring the samples recovery from of the a wet andsamples high-pressure from a wet seafloor and high-pressure environment, seafloor to a dry envi laboratoryronment, at to the a dry surface. laboratory Similar at post-recoverythe surface. alterationSimilar ispost-recovery echoed by the alteration desiccation is echoed cracks, by secondary the desiccation Na-carbonate cracks, secondary and Na-sulfate, Na-carbonate and probable and Na-sulfate, and probable pseudomorphic crystals seen in other layers of the black crusts. pseudomorphic crystals seen in other layers of the black crusts. Nano-particulate gold (Figure8C, Nano-particulate gold (Figure 8C, sub-micron-sized white specks) is rare but present in these sub-micron-sized white specks) is rare but present in these mineralized colonies. mineralized colonies. The second, and dominant morphology observed within the black crust middle layer has similar The second, and dominant morphology observed within the black crust middle layer has scale and clustering to live Spirilla filamentous biofilms grown on the surface of mild carbon steel [21]. similar scale and clustering to live Spirilla filamentous biofilms grown on the surface of mild carbon Thesteel Spirilla [21]. The bacteria Spirilla are bacteria elongate, are averaging elongate, averaging ten microns ten in microns length, in and length, arcuate, and arcuate, creating creating a dendritic a colony.dendritic Heavily colony. biomineralized Heavily biomineralized and branching and filamentous branching filamentous bacteria colonies bacteria (including colonies (includingC. metalidurans C. ) similarmetalidurans to these) similar have been to these observed have growingbeen observed on the grow surfaceing ofon placerthe surface gold nuggetsof placer(Figures gold nuggets3 and 4 in(Figures [23]; Figure 3 and5D 4 inin [23]; [ 14]). Figure Micron 5D in to [14]). sub-micron Micron to nano-particulate sub-micron nano-parti gold (whiteculate gold dots) (white is present dots) in significantis present quantities in significant throughout quantities this throughout biofilm type. this biofilm type.

4.2.2.4.2.2. Nanoparticulate Nanoparticulate Gold Gold and and Probable Probable GoldGold Sources TheThe nanoparticulate nanoparticulate gold gold thatthat occursoccurs inin the biomat layer layer has has a a range range of of alloy alloy compositions compositions (Figure(Figure7). 7). Loose Loose material material from from a a crust crust withwith highhigh ironiron nanoparticuatenanoparticuate gold gold was was sonicated sonicated and and

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re-analyzed to reveal a minimal iron content, suggesting that the iron is a weakly-bound surficial coating or dusting. As a result, iron is not included as a major or minor element in this reporting. Significantly, only one of these analyses is a good match for the composition of the alloy of the gold coins on which they are growing. The 1857-S US $20 gold coins on which the black crusts formed is an alloy of 90% gold, with 10% copper to add durability and wear resistance [18]. Relative solubilities suggest that copper in the alloy would preferentially corrode significantly faster than the gold as a depletion-gilding effect [26]. Yet, only 9% of the nanoparticulate gold has a composition J.consistent Mar. Sci. Eng. with2019 this, 7, 209 source, which would be more copper-rich than the presumed coin source.11 More of 16 interestingly, 27% of the nanoparticulate gold has significant quantities of silver, which is not re-analyzedcontained in to the reveal alloy a minimalof the US iron $20 content, gold suggestingcoins beyond that thetraces. iron The is a weakly-boundremaining 64% surficial of the coatingnanoparticulate or dusting. gold As that a result, was analyzed iron is not has included a composition as a major richer or minorin gold element than the in coin this substrate. reporting. Significantly,Potential sources only of one gold, of thesesilver, analyses and copper is a on good the matchSS Central for theAmerica composition shipwreck of theinclude alloy several of the goldtons coinsof US on gold which and they silver are coins, growing. unrefined The 1857-S assay US ingots, $20 gold and coins natural on which placer the gold. black It crustsalso includes formed isseveral an alloy more of 90% tons gold, of copper with 10% anti-foulin copperg to hull add sheathing, durability and wearbrass resistancefittings. [18]. Relative solubilities suggestSilver that in copper the nanoparticulate in the alloy would gold preferentiallycannot be orig corrodeinating significantlyfrom a gold fastercoin source, than the as gold it is asnot a depletion-gildingfound in the coin e ffalloyect [26 beyond]. Yet, onlythe 9%traces of thepermitted nanoparticulate by the US gold Mint has apolicy composition and federal consistent law. withFurthermore, this source, the which lack of would micro-pitti be moreng copper-richand preservation than the of presumed mirror-like coin finishes source. (Figure More interestingly,9) speaks to 27%insignificant of the nanoparticulate corrosion of the gold coins. has significant It is also unlikely quantities that of silver,the silver which comes is not from contained proximity in the to alloy US ofsilver the coins US $20 found gold on coins the beyond shipwreck, traces. as Thesilver-r remainingich and 64%silver-poor of the nanoparticulatenanoparticulate goldgold that is often was analyzedfound within has aa composition few microns richer of each in goldother than in the the same coin substrate.black crust sample. Similarly, proximity to copperPotential sheathing sources on ofthe gold, hull silver, cannot and explain copper onthe the distribution SS Central Americaof copper-shipwreckrich and include copper-poor several nanoparticulate gold. tons of US gold and silver coins, unrefined assay ingots, and natural placer gold. It also includes However, assay ingots and placer gold from the shipwreck do provide a plausible source for several more tons of copper anti-fouling hull sheathing, and brass fittings. both the copper and silver in the nanoparticulate gold, given the enrichments expected through Silver in the nanoparticulate gold cannot be originating from a gold coin source, as it is not found depletion-gilding release of metals from the source (Figure 7). Assay ingots from the shipwreck in the coin alloy beyond the traces permitted by the US Mint policy and federal law. Furthermore, average 88% gold, 11% silver, and 1% copper, while the placer gold averages 89% gold, 10.5% silver, the lack of micro-pitting and preservation of mirror-like finishes (Figure9) speaks to insignificant and 0.5% copper, based upon EDS analyses and values stamped on ingots recovered from the corrosion of the coins. It is also unlikely that the silver comes from proximity to US silver coins found shipwreck. It is expected that the ingots would be similar in composition to the placer gold, as the on the shipwreck, as silver-rich and silver-poor nanoparticulate gold is often found within a few ingots are chiefly melted bars of unrefined placer gold. The slightly lower purity of the assay bars microns of each other in the same black crust sample. Similarly, proximity to copper sheathing on the likely results from the inclusion of small amounts of transition metal enriched heavy sands included hull cannot explain the distribution of copper-rich and copper-poor nanoparticulate gold. with the placer gold when melted.

Figure 9.9. ((AA)) TheThe proof-likeproof-like finish finish of of an an 1857-S 1857-S US US $20 $20 gold gold coin coin recovered recovered from from the the shipwreck shipwreck can stillcan createstill create sharp sharp reflections reflections of text of after text157 after years 157 onyears the seafloor.on the seafloor. (B) SEM (B imaging) SEM imaging of gold coins of gold from coins the wreckfrom the reveal wreck a sharp reveal mint-like a sharp mint-like state with state no evidence with no ofevidence corrosion of corrosion or pitting. or pitting.

However,The observed assay range ingots of both and placersilver-rich gold and from coppe the shipwreckr-rich nanoparticulate do provide gold a plausible is possible source by the for bothphysical the process copper of and depletion silver in gild theing, nanoparticulate but not simultaneously. gold, given There the enrichmentsare two possibilities expected that through would depletion-gildingenable this to occur. release First, of if metals the conditions from the sourceof leaching (Figure of 7placer). Assay and ingots assay from ingot the gold shipwreck were to average change 88% gold, 11% silver, and 1% copper, while the placer gold averages 89% gold, 10.5% silver, and 0.5% copper, based upon EDS analyses and values stamped on ingots recovered from the shipwreck. It is expected that the ingots would be similar in composition to the placer gold, as the ingots are chiefly melted bars of unrefined placer gold. The slightly lower purity of the assay bars likely results from the inclusion of small amounts of transition metal enriched heavy sands included with the placer gold when melted. The observed range of both silver-rich and copper-rich nanoparticulate gold is possible by the physical process of depletion gilding, but not simultaneously. There are two possibilities that would J. Mar. Sci. Eng. 2019, 7, 209 12 of 16 enable this to occur. First, if the conditions of leaching of placer and assay ingot gold were to change significantly, it could produce a significant change in gilding effluent. However, given the buffering capacity of the ocean, such an extreme environmental swing is considered unlikely. The other mechanism that could produce the range of disparate nanoparticulate gold alloy compositions is biological fractionation. If depletion gilding of placer and assay ingot gold produces a fairly uniform effluent chemistry, it is possible that two populations of metal-tolerant microbes could precipitate different nanoparticulate gold alloys through different metabolic pathways. Cupriavidus metallidurans are known to dominate nanoparticulate gold production in the biofilms on placer gold [27,28]. However, a wide range of bacteria is known to be capable of precipitation of gold nanoparticles [29]. It has also been shown that two bacteria capable of gold biomineralization, Cupriavidus metallidurans and Delftia acidovorans, can co-exist within biofilms [30]. The ability of these microbes to detoxify their environment through gold nanoparticle formation has been linked to copper resistance [31], which may be linked to the amounts of copper within the gold alloy. Given that the black crusts have two distinct biomineralization styles, and two distinct nanoparticulate gold alloy populations, it is certainly possible that the range of gold particle chemistry reflects biological fractionation. This biological accumulation of gold is known to occur over the decadal-scale [11], consistent with formation in the black crusts from the shipwreck. The ability of these types of bacteria to compete with sea floor bacterial communities; and their tolerance of cold, saline, high-pressure environments is unknown and should be examined by future workers. Nanoparticulate gold has been shown to form through reduction by residual organic material such as humic acids and extracellular polymeric substances [11]. Such materials should be abundant around the shipwreck site, due to degradation of abundant wood, cloth, and stored food. Ferric iron may also play a key role as a reducing agent and the precipitation of gold. It is possible that one or both of the nanoparticulate gold populations results from these process. Regardless of the exact combination of mechanisms, it is likely that the range of nanoparticulate gold chemistry reflects two mechanisms, at least one of which is biological.

4.3. Chronology and Significance of Crust Development Creation of black crusts on the gold coins from the shipwreck of the SS Central America began the moment the ship slipped beneath the waves on 12 September, 1857. The large quantity of mild carbon steel and cast iron on the ship was the first material to react, due to its high corrosion potential relative to other metals [32]. Corrosion of these types of metals has been studied at several deep shipwrecks and in the laboratory. Corrosion studies from the RMS Titanic and ships that sank during World War II indicate that significant corrosion may take place during the first couple decades [33–35]. Specifically, studies of the shipwreck of the nuclear submarine Komsomolets suggest corrosion rates up to 75 microns/year [36], while corrosion conditions of the tanker Prestige are believed to be about 57 microns/year [37]. It is known that these corrosion rates change over time, and multiple studies have shown that corrosion peaks within the first couple years to decades, and tail off to a lower steady state in the period that follows [38]. This dissolution of SS Central America’s iron, and subsequent reaction with seawater to form a fine-grained precipitate of black to red iron oxides, is expected from evidence at other shipwreck sites [39]. Thus, it seems reasonable that the black crusts initially formed as a fine-grained laminar deposit on the coins, as a chemical process that occurred within the first decade after the sinking. It is known that the formation of rusticles, or porous icicle-like rust growths on deep-sea shipwrecks, is a biological process [33,34,40]. It was in this later stage of shipwreck corrosion that biology began to dominate. These bacteria come from seawater, and are not known to produce nanoparticulate gold. In fact, aqueous gold and silver is highly toxic to most bacteria [27,28]. Only specific bacteria, such as Cupriavidus metallidurans and Delftia acidovorans, have developed tolerance to these metals and can exist within biofilms on gold [30]. However, the localized Fe3+ concentrations around the shipwreck at this time would have selected for enrichment of natural iron oxidizing bacteria and may have accelerated J. Mar. Sci. Eng. 2019, 7, 209 13 of 16 the rate of iron oxidation. As the shipwreck was slowly colonized by rusticle-producing bacteria, this may have added another source of iron precipitate that could filter onto the coins to add to the laminar layer of the crust. Specifically, rusticles have a lifecycle of colonization, development, and collapse of these fragile structures [34]. Such cycling would produce abundant “rusticle dust,” which could further coat the coins when physically remobilized during shipwreck collapse and benthic storms. At this point, two biological environments were formed. The first is the consortia of bacteria that colonized the iron and steel of the site and contributes to rusticle formation. Members of this consortia could not establish colonies on the gold in the shipwreck, due to precious metal toxicity. Later, the armoring of the gold coins and some assay bars by laminar geochemical precipitates and “rusticle dust” black crusts produced a safer low aqueous gold environment for bacteria to live. This leads to the second consortia of bacteria in the black crusts. Freshly mined natural placer gold was a likely source of gold-tolerant bacteria such as Cupriavidus metallidurans and Delftia acidovorans, which have developed tolerance to gold [30]. It has been shown that these bacteria are capable of survival under less than ideal post-mining conditions for at least 18 months [14]. Assay bars and coins, which have been processed, would have been sterilized of these bacteria. However, following the sinking and reintroduction to an aqueous environment, these natural colonies on the placer gold would thrive and remobilize to precipitate gold as part of their life cycle [30]. In the moderately gold-rich niche-environment of the black crusts, these bacteria would be able to out-compete native ocean-sourced Fe and Mg precipitating bacteria. However, by the time these bacteria could spread to other parts of the site the coins and bars of gold had their armoring of abiological laminar black crusts that protected them from direct attack and precipitation of gold on the coins themselves. A second possibility is that iron oxides played a role in reduction of nanoparticulate gold upon previously existing iron microfossils. As the biological colonization of the black crusts continues, these bacteria produce waste products from their local environment, including nanoparticulate gold and base metal sulfides. They also produce significant quantities of metabolic wastes such as oxygen, carbon dioxide, and sulfur, which form an “atmosphere” of enrichment around the colonies. These elements react with the dissolved iron from the ship to produce iron oxides, iron carbonate, and iron sulfates. These form larger (up to 25 micron) euhedral crystals because of the low saturation and long period of reaction (Figure5D). These black crusts appear to have played a key role in the exquisite state of preservation of the gold coins (Figure9). Gold-precipitation bacteria are clearly present in the materials of the shipwreck site. In natural environments, as little as 7.6 years are required to produce cycles of gold dissolution and re-precipitation, which can produce spectacular structures both into and onto the gold substrate [11]. Similarly, a 1908 $20 gold coin recovered from gravels of the Fairbanks, Alaska placer district in 1959 display crystals up to 1 mm of secondary gold firmly attached to the surface of the coin [41]. Such activity would be expected on the gold of the SS Central America shipwreck, and would have a profound effect on gold coin preservation that would be visible even to the naked eye. However, this did not occur. The degree of preservation of these coins is ascribed to two main factors. First, compartmentalization of the gold within the shipwreck inhibited migration of bacteria that would have degraded the coins. Second, the enormous quantities of iron on the ship quickly yielded a geochemical coating that armored these coins from future attack as a passivation layer when the shipwreck degraded and gold-metabolizing bacteria moved from their placer gold host to other parts of the site. This not only prevents corrosion of the coins, but also prevents gold from being dispersed by ocean currents.

5. Conclusions Black crusts that formed on gold coins recovered from the 1857 shipwreck of the SS Central America played a key role in preservation of gold artifacts from this important archeological site. While it is expected that there would be degradation of the quality of the coins in this horde, they preserve a near mint-like to proof-like state. The significant quantities of iron and steel on the shipwreck produced a fine-grained iron mineral coating on the coins within a few years of the sinking. This coating armored the coins from future chemical or biological attack. Once coated, the coins were colonized by at least J. Mar. Sci. Eng. 2019, 7, 209 14 of 16 two distinct populations of gold-tolerant bacteria that precipitated abundant nanoparticulate gold in the black crust material and produced biomineralized bacteria in a web-like mat. This demonstrates that bacteria that are known to mobilize and re-precipitate gold were probably present in the shipwreck, likely being introduced from the unrefined placer gold in the cargo. Above this middle layer of black crust, the outer layer consists of a geochemical reaction front of iron minerals, formed by the interaction of seawater with the chemical wastes of the underlying bacterial mat. Understanding this process has application for assessing the diverse and extreme conditions under which nanoparticulate gold may form through biological processes. For example, while it is well known that iron metabolizing bacteria can thrive under high pressure and cold temperatures at deep sea shipwreck sites, this present study documents that the same is true for at least some members of the group of gold-tolerant bacteria, expanding their known environmental window of habitability. Similarly, this study helps with our understanding of the conditions that contribute to preservation or degradation of marine archaeological materials. It can help with predictive models of the degree of precious metal preservation at sites awaiting excavation, and potentially identify sites where the precious metal artifacts may be threatened and require priority excavation.

Author Contributions: Conceptualization, methodology, and analyses were performed by all authors; sample collection and preparation, R.E.; draft preparation, E.M.; writing—review and editing was performed by all authors. Funding: This research received no external funding. Instrumentation was funded by the W.M. Keck Foundation, and an internal grant from CSUSB. Acknowledgments: We thank California Gold Marketing Group and Professional Coin Grading Service (PCGS) for providing access to the materials that were used in this study, and additional materials that provided context for the conditions of the shipwreck site. We also thank the hard-working members of the 2014 expedition that recovered the materials used in the study. Instrumentation used in this study was provided by a generous grant from the W.M. Keck Foundation, and an internal VETI Grant by CSUSB. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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