Surface ocean iron (Fe) fertilization can affect the marine primary productivity (MPP), thereby impacting on CO2 exchanges at the atmosphere‐ocean interface and eventually on climate. Mineral (aeolian or desert) dust is known to be a major atmospheric source for the surface ocean biogeochemical iron cycle, but the significance of volcanic ash is poorly constrained. We present the results of geochemical experiments aimed at determining the rapid release of Fe upon contact of pristine volcanic ash with seawater, mimicking their dry deposition into the surface ocean. Our data show that volcanic ash from both subduction zone and hot spot volcanoes (n = 44 samples) rapidly mobilized significant amounts of soluble Fe into seawater (35–340 nmol/g ash), with a suggested global mean of 200 ± 50 nmol Fe/g ash. These values are comparable to the range for desert dust in experiments at seawater pH (10–125 nmol Fe/g dust) presented in the literature (Guieu et al., 1996; Spokes et al., 1996). Combining our new Fe release data with the calculated ash flux from a selected major eruption into the ocean as a case study demonstrates that single volcanic eruptions have the potential to significantly increase the surface ocean Fe concentration within an ash fallout area. We also constrain the long‐term (millennial‐scale) airborne volcanic ash and mineral dust Fe flux into the Pacific Ocean by merging the Fe release data with geological flux estimates. These show that the input of volcanic ash into the Pacific Ocean (128–221 × 1015 g/ka) is within the same order of magnitude as the mineral dust input (39–519 × 1015 g/ka) (Mahowald et al., 2005). From the similarity in both Fe release and particle flux follows that the flux of soluble Fe related to the dry deposition of volcanic ash (3–75 × 109 mol/ka) is comparable to that of mineral dust (1–65 × 109 mol/ka). Our study therefore suggests that airborne volcanic ash is an important but hitherto underestimated atmospheric source for the Pacific surface ocean biogeochemical iron cycle.

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The Holocene eruptive history of Popocatépetl volcano is characterized by recurrent voluminous Plinian eruptions every 1000 to 3000 yr, the most recent of which destroyed human settlements. Major eruptions occurred between 3195 and 2830 B.C., 800 and 215 B.C., and A.D. 675 and 1095. The three eruptions followed a similar pattern and started with minor ash fall and ash flows. The eruptions reached their peak with a main Plinian pulse that produced deposition of a pumice fall, the emplacement of hot ash flows, and finally extensive mudflows. Each time the area of devastation had become repopulated, before being devastated once again. During the last eruption several settlements, including Cholula (a major urban center), were inundated by lahars. A scenario of the possible recurrence of an eruption of similar magnitude, which would have disastrous consequences for the now highly populated areas around Popocatépetl, should be considered seriously in any volcano emergency contingency plan. This is especially important because more than one million people are living within a radius of 35 km around the volcano (the outskirts of Mexico City are at a distance of 40 km), and Popocatépetl resumed emitting ash on December 21, 1994, after decades of dormancy.

The Pedregal lavas are fresh, well-exposed basaltic¯ows erupted from the Xitle scoria-and-cinder cone in the southwestern part of the Basin of Mexico. These lavas cover an area of 70 km 2 and were emplaced over pyramids and other buildings (e.g. Cuicuilco and Copilco archaeological sites). Today, a part of Mexico-City (including the National University) is built on thē ows. Initial strombolian activity produced an ash fallout layer, which was immediately followed by effusive emplacement of lavā ows. The Xitle cone grew on the north-facing slope of Ajusco volcano, and lava¯owed down to the N±NE until it reached the basin¯oor.More than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava, and several ages for the eruption have been proposed from these dates. Most dated samples were not directly produced by Xitle's eruption but instead are artifacts of human activity that predates the eruption. Thus, these ages (mostly about 2000 bp) are older than the eruption. A new age of 1670^35 years bp (AD 245±315) obtained on charcoal samples collected just beneath the lavas is favored for the Xitle eruption. These samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra. The new age is within the Classic period of Mesoamerican archaeology, whereas the earlier reported ages are at the end of the Preclassic. The new age carries important implications for the timing of population shifts within the Basin of Mexico. q

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Pelado, Guespalapa, and Chichinautzin monogenetic scoria cones located within the Sierra del Chichinautzin Volcanic Field (SCVF) at the southern margin of Mexico City were dated by the radiocarbon method at 10,000, 2,800-4,700, and 1,835 years b.p., respectively. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco around 1,665€35 years b.p. The new dates indicate that the recurrence interval for monogenetic eruptions in the central part of the SCVF and close to the vicinity of Mexico City is <2,500 years. If the entire SCVF is considered, the recurrence interval is <1,700 years. Based on fieldwork and Landsat imagery interpretation a geologic map was produced, morphometric parameters characterizing the cones and lava flows determined, and the areal extent and volumes of erupted products estimated. The longest lava flow was produced by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each eruption range between 54 (Chichinautzin) and 80 km 2 (Pelado); and total erupted volumes range between 1 and 2 km 3 /cone. An average eruption rate for the entire SCVF was estimated at 0.6 km 3 /1,000 years. These findings are of importance for archaeological as well as volcanic hazards studies in this heavily populated region.

The Zacapu lacustrine basin is located in the north-central part of the Michoacán-Guanajuato volcanic field (MGVF), which constitutes the west-central segment of the Trans-Mexican Volcanic Belt. Geological mapping of a 395 km 2 quadrangle encompassing the western margin of the basin, 40 Ar/ 39 Ar and 14 C radiometric dating, whole-rock chemical and petrographic analyses of volcanic products provide information on the stratigraphy, erupted volumes, age, and composition of the volcanoes. Although volcanism in the MGVF initiated since at least 5 Ma ago, rocks in the western Zacapu lacustrine basin are all younger than ~2.1 Ma. A total of 47 volcanoes were identified and include 19 viscous lava flows (~40 vol.%), 17 scoria cones with associated lava flows (~36 vol.%), seven lava shields (~15 vol.%), three domes (~6 vol.%), and one maar (~2 vol.%). Erupted products are dominantly andesites with 42 km 3 (~86 vol.%) followed by 4 km 3 of dacite (~8 vol.%), 1.4 km 3 of basaltic trachyandesite (~3 vol.%), 1 km 3 of basaltic andesite (~2 vol.%), and 0.14 km 3 of rhyolite (~0.3 vol.%). Eruptive centers are commonly aligned ENE-WSW following the direction of the regional Cuitzeo Fault System. Over time, the high frequency of eruptions and consequent accumulation of lavas and pyroclastic materials pushed the lake's shore stepwise toward the southeast. Eruptions appear to have clustered through time. One cluster occurred during the Late Pleistocene between ~27,000 and 21,300 BC when four volcanoes erupted. A second cluster formed during the Late Holocene, between ~1500 BC and ~AD 900, when four closely spaced monogenetic vents erupted forming thick viscous 'a'a to blocky flows on the margin of the lacustrine flats. For still poorly understood reasons, these apparently inhospitable lava flows were attractive to human settlement and eventually became one of the most densely populated heartlands of the pre-Hispanic Tarascan civilization. With an average eruption recurrence interval of ~900 years during the Late Holocene the western Zacapu lacustrine basin is one of the most active areas in the MGVF and should hence be of focal interest for regional volcanic risk evaluations.Keywords Zacapu basin . Michoacán-Guanajuto volcanic field . Quaternary . Monogenetic cluster . Radiocarbon . 40 Ar/ 39 Ar dating . Malpaís * Nanci Reyes-Guzmán

1977±1978 (ref. 23). A re-evaluation of systematic errors between GEOSECS and I8NR data is made by comparing deep-water values to determine the appropriate corrections before making the direct comparison for detecting the anthropogenic carbon signal.To determine the mean systematic difference between GEOSECS and I8NR, we compared samples from deeper than 2,000 m. First, the I8NR stations are organized into 6 groups corresponding to 6 stations occupied during the GEOSECS. Each group covers a range of 58 latitudes, with the centre location representing the re-occupation of GEOSECS stations. The mean concentration is computed for samples taken at 8 isopycnal surfaces and j v ranges from 27:75 6 0:01 to 27:82 6 0:01, which cover all samples from deeper than 2,000 m. The concentration difference along the same isopycnal surface at the same location between the two cruises is then computed. Results of the comparison are given in Table 1.We corrected the natural variations of DIC before making the intercomparison. The total carbon released into the water by the respiration of organic matter is estimated using the Red®eld ratio 12 C:O 2 of 117 6 14:170 6 10. The AOU-corrected DIC (DIC aou ) can be given as DIC aou DIC j 2 0:69 3 AOU, where DIC j is the observed DIC on an isopycnal surface. The choice of this Red®eld ratio is based on newer published results and has an insigni®cant effect on the magnitude of the detected CO 2 signal. If a ratio of 140/172, as derived from Indian Ocean 24 , is used, the signal will change by ,1 mmol kg -1 because the anthropogenic DIC increase is derived by difference of two data sets in which the same ratio is applied.The effect of changes in carbonate dissolution at two different times can be corrected by using Alk data. The DIC corrected for Alk (DIC alk ) is given by DIC alk DIC aou 2 0:5 3 Alk 2 Alk 0 , where Alk is the observed alkalinity for the isopycnal surface, and Alk 0 is the preformed salinity-normalized Alk, which can be calculated by using the empirical equation 25 : Alk 0 2;291 2 2:52t 2 20 0:06t 2 20 2 , where t is the potential temperature at the isopycnal horizon.To eliminate changes caused by the variations of salt, the corrected DIC is normalized to a salinity of 35.0: DIC n DIC alk =S 3 35:0, where DIC n is the salinity-normalized ®nal DIC value used for comparison between two different cruises.For estimating DIC increase from atmospheric CO 2 increase, the following needs to be considered. The potential temperature for waters at isopycnal surfaces between 26.6 and 27.2 ranges from 8 to 12 8C in the equatorial region and 6 to 13 8C in the temperature zone. Taking a mean potential temperature of 10 8C for this upper thermocline water, and a mean surface water alkalinity of 2,290 mmol kg -1 , the DIC at chemical equilibrium with the atmospheric CO 2 can be calculated. The rate of atmospheric CO 2 increase varies from 0.8 p.p.m. yr -1 in the early 1960s to ,1.8 p.p.m. yr -1 in recent years 26,27 . The DIC increase is computed using the rate of atmospheric CO 2 increase...