STROMATOLITES LAMINATED LITHIFIED STRUCTURES TYPICALLY BUILT BY MICROBIAL MATS REPRESENT THE MOST ABUNDANT RECORD OF LIFE IN THE FIRST 7/8THS OF EARTH HISTORY (THE ARCHEAN AND PROTEROZOIC EONS). AS MACROSCOPIC MANIFESTATIONS OF MICROBIAL LIFE THEY REPRESENT A CLEAR TARGET FOR EXOBIOLOGIC INVESTIGATION. DESPITE A LONG HISTORY OF STUDY MANY STRIKINGLY FUNDAMENTAL QUESTIONS REMAIN WITH RESPECT TO STROMATOLITE FORMATION AND THEIR DISTRIBUTION THROUGHOUT GEOLOGIC TIME. TWO PERTINENT QUESTIONS INCLUDE: 1) HOW DO MICROBIAL MATS LITHIFY TO BECOME STROMATOLITES (I.E. ROCKS)? IT IS STILL NOT CLEAR HOW SOFT ORGANIC-RICH MICROBIAL MATS TRANSFORM INTO HARD LITHIFIED STROMATOLITES. 2) WHAT CONTROLLED THE DISTRIBUTION OF STROMATOLITES THROUGH TIME? STROMATOLITES REACHED A FORM DIVERSITY PEAK CA. 1.0 BILLION YEARS AGO CRASHED AND BECAME COMPARATIVELY RARE THROUGHOUT THE REMAINING 600 MILLION YEARS OF EARTH HISTORY. THE MOST COMMON HYPOTHESIS THE EVOLUTION OF BURROWING/GRAZING METAZOANS DISRUPTING THE MICROBIAL MATS IS NOT TENABLE AS THE DECLINE INITIATED WELL BEFORE THE ADVENT OF ANIMALS. HERE WE PROPOSE TO INVESTIGATE HOW THE REDOX CYCLING OF IRON NOT PREVIOUSLY CONSIDERED IMPORTANT FOR THE FORMATION OF STROMATOLITES MAY LEAD TO THE LITHIFICATION OF SOFT MICROBIAL MATS AND HOW THE CHANGES IN THE DISSOLVED IRON IN ARCHEAN PROTEROZOIC AND PHANEROZOIC OCEANS MAY LINK TO THE DECLINE IN STROMATOLITE ABUNDANCE AND FORM DIVERSITY WELL BEFORE THE EVOLUTION OF ANIMALS. IRON LITHIFICATION ANCIENT OCEAN CHEMISTRY AND MATS: BEFORE WIDESPREAD MARINE OXYGENATION THE OXYGEN PRODUCED LOCALLY BY CYANOBACTERIA IN A MICROBIAL MAT COULD INDUCE IRON OXIDE PRECIPITATION IN THE MAT DURING SUNLIGHT HOURS. AT NIGHT WHEN THE MAT BECAME ANOXIC IRON-REDUCING BACTERIA COULD USE THE IRON OXIDES TO RESPIRE RELEASING IRON INTO PORE WATERS. DURING MICROBIAL IRON REDUCTION A BOOST OF ALKALINITY IS PROVIDED MORE THAN ANY OTHER COMMON METABOLISM SHIFTING THE LOCAL GEOCHEMICAL ENVIRONMENT IN FAVOR OF CALCIUM CARBONATE PRECIPITATION IN THE MAT. TESTABLE CONSEQUENCES ARE EVIDENT INCLUDING A PREDICTED CYCLIC IRON ENRICHMENT WITHIN STROMATOLITES FORMED BY THIS PROCESS AND A LINK BETWEEN STROMATOLITE ABUNDANCE/DECLINE AND MARINE IRON CONCENTRATIONS THROUGH GEOLOGIC TIME. IN TESTING THE ROLE OF IRON REDOX CYCLING IN THE LITHIFICATION OF STROMATOLITES WE MAY FINALLY UNDERSTAND THE DECLINE OF STROMATOLITE FORM DIVERSITY/ABUNDANCE THAT HAS REMAINED UNRESOLVED SINCE IT WAS RECOGNIZED. THE NOVEL LINK BETWEEN MICROBIAL IRON REDUCTION AND STROMATOLITES WILL BE TESTED IN TWO WAYS: 1. BY EXAMINING IRON REDUCTION IN MODERN STROMATOLITES GROWING IN OBSIDIAN POOL PRIME (OPP) A SILICEOUS HOT SPRING IN YELLOWSTONE NATIONAL PARK. HERE A SUITE OF MODERN MOLECULAR (SSU RRNA METAGENOMIC/TRANSCRIPTOMIC) GEOCHEMICAL AND IMAGING TOOLS WILL BE USED TO INVESTIGATE THE IMPORTANCE OF IRON REDUCTION IN A TRACTABLE SYSTEM. ALTHOUGH OPP IS A SILICA SPRING THE INFORMATION GAINED IS APPLICABLE TO OTHER SYSTEMS INCLUDING CARBONATE SYSTEMS. SINCE THE GROWTH RATE OF THE OPP STROMATOLITES IS KNOWN THE AFOREMENTIONED APPROACHES WILL BE CAPTURED FOR THE FIRST TIME DURING STROMATOLITE LAMINATION FORMATION/GROWTH. 2. BY USING MICRO X-RAY FLUORESCENCE TECHNIQUES TO INVESTIGATE THE DISTRIBUTION OF IRON WITHIN A SUITE OF STROMATOLITES THROUGH THE RISE AND FALL OF STROMATOLITE ABUNDANCE IN GEOLOGIC TIME TESTING THE PREDICTION OF IRON INCORPORATION DURING LAMINATION FORMATION VERSUS MARINE IRON CONCENTRATIONS. COINCIDENTALLY THE DECLINE OF STROMATOLITES NEATLY TRACKS THE LOSS OF THE FERRUGINOUS OCEANS AS INDICATED BY MO AND CR PROXIES LENDING CREDENCE TO THE HYPOTHESIS. OUR STUDY WOULD BE RELEVANT TO TWO MAIN FOCI IN THE EXOBIOLOGY PROGRAM 1) EARLY EVOLUTION OF LIFE AND THE BIOSPHERE (STROMATOLITES REPRESENT THE EARLIEST MACROSCOPIC EVIDENCE OF LIFE ON EARTH) AND 2) BIOSIGNATURE AND LIFE ELSEWHERE (STROMATOLITES COULD ACT AS BIOSIGNATURES FOR LIFE ON EARLY EARTH/ELSEWHERE).