Mixed-layer Species Research Articles

Indian monsoon wind variability since ~11 kyr in the northwestern and northeastern Arabian Sea

• We studied varying monsoon wind stress in the Arabian Sea during the Holocene . • Surface productivity was high during the early Holocene . • Summer monsoon was weak and winter monsoon was strong in the middle Holocene. • Monsoon winds show high spatial variability in strength in the Arabian Sea. In this paper, we use planktic foraminifera and mixed-layer species from cores SK-240/327, SK-243/I- and ABP-25/03, northeastern Arabian Sea to investigate monsoon wind variability. Planktic foraminifera include Globigerina bulloides, a monsoon proxy and mixed-layer species Globigerinoides ruber, Gs. sacculifer, Gs. obliquus and Gs. extremus from cores SK-240/327, SK-243/I-1 and ABP-25/03 encompassing last ~11 calibrated kilo years before the Present (~11 cal kyr BP). The population abundances of Globigerina bulloides and mixed-layer species were compared with published Globigerina bulloides percentages from the western Arabian Sea Ocean Drilling Program Holes 723A, 724B and 727B, and eastern Arabian Sea core ABP-25, 02 to understand variations in Indian monsoon winds during the Holocene. In the eastern Arabian Sea, upwelling induced productivity was higher during 10–6.5 cal kyr BP which decreased during 6.5–3 cal kyr BP. In the northwestern Arabian Sea, we observe a similar scenario but not every site captured this change owing to spatial diverseness in monsoon wind stress. The productivity increased since 3 cal kyr BP due to intense summer monsoon winds in the northeastern Arabian Sea, whereas productivity began to increase at ~1.5 cal kyr BP in the northwestern Arabian Sea. The upwelling intensity shows large spatial and temporal variations in the eastern and western Arabian Sea, indicating varying monsoon wind strength across the Arabian Sea during the Holocene. The spectral analysis performed on Globigerina bulloides shows cyclicity which is associated with solar variability during the past 11 kyr record in the northeastern Arabian Sea.

Paleoceanographic turnovers during the Plio-Pleistocene in the southeastern Indian Ocean: Linkages with Northern Hemisphere glaciation and Indian Monsoon variability

Earth experienced warm temperatures during the Pliocene followed by a substantial cooling that culminated in an ice age climate at the beginning of the Pleistocene. This warmth had a significant impact on global environments and ocean circulation. The collision of the Indian and Eurasian plates further amplified this climatic pattern. The southern Indian Ocean was severely impacted by these changes in ocean circulation and climatic pattern during the Plio-Pleistocene. We reconstruct paleoceanographic turnovers in the southeastern Indian Ocean during the Plio-Pleistocene using benthic foraminiferal assemblages from ODP Sites 752 and 757, combined with relative abundances of selected planktic foraminiferal species and stable isotope ratios (δ18O and δ13C) of surface-dwelling and thermocline planktic foraminifera from Site 757. The influence of southern source Antarctic Intermediate Water (AAIW) since 4.5 Ma is marked by fluctuations in productivity till the intensification of the Northern Hemisphere glaciation (NHG) at ~3 Ma. Our isotopic data provide no evidence for cooling during the 4–3 Ma interval either for upper (Gs. ruber) or lower (M. menardii) mixed-layer waters, at a time when a switch in the source of the Indonesian Throughflow may have occurred. The increased mixed-layer species abundances since 4 Ma suggest oligotrophic conditions at low-latitude Site 757 due to thick mixed-layer coinciding with the constriction of the Indonesian Gateway and intensification of the NHG. The intensification of the NHG and monsoon seasonality caused an enhanced inflow of AAIW in the southeastern Indian Ocean since ~2.5 Ma.

Evolution of the Oligotrophic West Pacific Warm Pool During the Pliocene‐Pleistocene Boundary

AbstractThis study investigates the development of oligotrophic conditions, thickening, and zonal and meridional contraction of the West Pacific Warm Pool (WPWP) during the Pliocene. It has been hypothesized that the evolution of the WPWP and the establishment of strong equatorial Pacific zonal gradients are closely related to the narrowing of the Indonesian Gateway (IG) as well as the closure of the Central American Seaway (CAS). However, the timing of the development of these events remains unclear. Here we report Pliocene‐to‐Recent relative abundances of planktic foraminifera at Deep Sea Drilling Project (DSDP) Site 214 in the eastern Indian Ocean and at Ocean Drilling Program (ODP) Site 807, in the western Pacific. A comparison of the abundance of mixed‐layer species (MLS) from both sites indicates a pronounced increase in their population between ~3.15 and 1.6 Ma. There is a contemporaneous decrease in the Globigerinita glutinata population during this time, which together with the MLS data suggest the development of oligotrophic conditions in the western tropical Pacific. Our data suggest that the oligotrophic WPWP, resembling present‐day conditions, developed around 3.15 Ma and was closely linked to the gradual constriction of the IG.

Calcification depths and temperatures of planktonic foraminifera off southwest Hainan Island and their paleoceanographic implications

Abstract The East Asian monsoon system influences the local oceanographic and climatic conditions of South China Sea (SCS), making the habitat depths of planktonic foraminifera (PF) specific for this region.The previous paleo-ocean reconstructions of SCS have been based on calcification habitat depths derived from other oceans, which may have affected the accuracy of interpretations. In this study, the calcification depths of Globigerinoides ruber, Globigerinoides sacculifer, and Neogloboquadrina dutertrei from sediment traps deployed at 1000 m depth off southwest Hainan Island from July 2012 to April 2013 were analyzed with regard to the stable isotopes of oxygen (δ18O) and carbon (δ13C) to calculate their regional apparent calcification depths (ACDs) with the overall aim to improve the reconstruction of the upper ocean thermal structure. ACDs and temperature estimated from foraminifera δ18O ranged from 0 to 42 m and 25°C for G. ruber, 30-65 m and 23.3°C for G. sacculifer, and 75-100 m and 20.3°C for N. dutertrei, respectively. The difference between the thermocline-species (N. dutertrei) and mixed-layer species (G. ruber and G. sacculifer) δ18O (∆δ18OTh–ML) and the temperature (∆T) shows water column stratification. The periods of lower and higher ∆T and ∆δ18O were in phase with the changes in regional overlying wind speed. Additionally, this study reveals that N. dutertrei lives and calcifies its shells within the base of the mixed-layer depth (MLD) and upper thermocline depth in SCS. Overall, the results from this study provide a new insight to understand the mixed-layer and thermocline depth in the northern SCS.

Constraining foraminiferal calcification depths in the western Pacific warm pool

Highlights • First apparent calcification depth assessment of living foraminifera in the SE WPWP • Deep surface mixed layer causes deep apparent calcification depths. • Deep-dwelling G. hexagonus traces nutrient conditions in equatorial water masses. Abstract Insight into past changes of upper ocean stratification, circulation, and nutrient signatures rely on our knowledge of the apparent calcification depth (ACD) and ecology of planktonic foraminifera, which serve as archives for paleoceanographic relevant geochemical signals. The ACD of different species varies strongly between ocean basins, but also regionally. We constrained foraminiferal ACDs in the Western Pacific Warm Pool (Manihiki Plateau) by comparing stable oxygen and carbon isotopes (δ18Ocalite, δ13Ccalcite) as well as Mg/Ca ratios from living planktonic foraminifera to in-situ physical and chemical water mass properties (temperature, salinity, δ18Oseawater, δ13CDIC). Our analyses point to Globigerinoides ruber as the shallowest dweller, followed by Globigerinoides sacculifer, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and Globorotaloides hexagonus inhabiting increasing greater depths. These findings are consistent with other ocean basins; however, absolute ACDs differ from other studies. The uppermost mixed-layer species G. ruber and G. sacculifer denote mean calcification depths of ~ 95 m and ~ 120 m, respectively. These Western Pacific ACDs are much deeper than in most other studies and most likely relate to the thick surface mixed layer and the deep chlorophyll maximum in this region. Our results indicate that N. dutertrei appears to be influenced by mixing waters from the Pacific equatorial divergence, while P. obliquiloculata with an ACD of ~ 160 m is more suitable for thermocline reconstructions. ACDs of G. hexagonus reveal a deep calcification depth of ~ 450 m in oxygen-depleted, but nutrient-rich water masses, consistent to other studies. As the δ13C of G. hexagonus is in near-equilibrium with ambient seawater, we suggest this species is suitable for tracing nutrient conditions in equatorial water masses originating in extra-topical regions.

Evaluating controls on planktonic foraminiferal geochemistry in the Eastern Tropical North Pacific

To explore relationships between water column hydrography and foraminiferal geochemistry in the Eastern Tropical North Pacific, we present δO18 and Mg/Ca records from three species of planktonic foraminifera, Globigerinoides ruber, Globigerina bulloides, and Globorotalia menardii, collected from a sediment trap mooring maintained in the Gulf of Tehuantepec from 2006–2012. Differences in δO18 between mixed-layer species G. ruber and G. bulloides and thermocline-dweller G. menardii track seasonal changes in upwelling. The records suggest an increase in upwelling during the peak positive phase of El Niño, and an overall reduction in stratification over the six-year period. For all three species, Mg/Ca ratios are higher than what has been reported in previous studies, and show poor correlations to calcification temperature. We suggest that low pH (7.6–8.0) and [CO32−] values (∼70–120 μmol/kg) in the mixed layer contribute to an overall trend of higher Mg/Ca ratios in this region. Laser Ablation Inductively Coupled Mass Spectrometry analyses of G. bulloides with high Mg/Ca ratios (>9 mmol/mol) reveal the presence of a secondary coating of inorganic calcite that has Mg/Ca and Mn/Ca ratios up to an order of magnitude higher than these elemental ratios in the primary calcite, along with elevated Sr/Ca and Ba/Ca ratios. Some of the samples with abnormally high Mg/Ca are found during periods of high primary productivity, suggesting the alteration may be related to changes in carbonate saturation resulting from remineralization of organic matter in oxygen-poor waters in the water column. Although similar shell layering has been observed on fossil foraminifera, this is the first time such alteration has been studied in shells collected from the water column. Our results suggest a role for seawater carbonate chemistry in influencing foraminiferal calcite trace element:calcium ratios prior to deposition on the seafloor, particularly in high-productivity, low-oxygen environments.

Orbital and suborbital variability in the equatorial Indian Ocean as recorded in sediments of the Maldives Ridge (ODP Hole 716A) during the past 444 ka

Abstract This study is aimed at understanding past 444 ka record of climate variability in the equatorial Indian Ocean using high resolution records of planktic and benthic foraminifera and pteropods from Ocean Drilling Program Hole 716A, Maldives Ridge, southeastern Arabian Sea. In total, 892 samples of 10 cm 3 volume from 444 ka old sequence were analysed at 1 cm intervals to generate census data of the foraminiferal fauna and pteropods. The percent and detrended time series of mixed-layer species Globigerinoides ruber and Globigerinoides sacculifer and thermocline species Neogloboquadrina dutertrei , benthic foraminifera Cymbaloporetta squammosa , Sphaeroidina bulloides and Uvigerina proboscidea , and pteropods from ODP Hole 716A reveal significant changes in wind intensity during the past 444 ka. An abrupt decrease in the Cymbaloporetta squammosa population at c. 300 ka (across MIS 8/9) suggests a weakening of equatorial wind intensity, which could be linked to Indian monsoon and may have driven pronounced changes in the oxygen minimum zone in the Maldivian region. These changes were contemporaneous with the Mid-Brunhes Climatic Event, the beginning of aridity in the Indonesian-Australian region and the onset of a humid phase in equatorial East Africa as observed in several oceanic and continental records. This strengthens a connection between equatorial Indian Ocean wind intensity, the Indian monsoon and Indonesian-Australian-African climates. Supplementary material: Percentages of benthic and planktic foraminifera and pteropods used in the present study are available at http://www.geolsoc.org.uk/SUP18413

Paleoceanography of the South China Sea since the middle Miocene: evidence from planktonic foraminifera

Late middle Miocene to Pleistocene planktonic foraminifera (PF) in fairly continuous hemipelagic sediments recovered at Ocean Drilling Program (ODP) Site 1143 from the southern South China Sea (SCS) were studied with sampling intervals of 10 to 150 cm. A total of 16 PF events are revised for the 516-m long section, providing an age model for the last 12 Ma at this site. On the basis of benthic foraminiferal δ 18O stratigraphy and the smoothed planktonic age model, new ages are assigned to three PF events (last occurrence [LO] Globorotalia multicamerata sensu lato [s.l.] at 2.18 Ma, first occurrence [FO] Sphaeroidinella dehiscens sensu stricto [s.s.] at 3.6 Ma, and FO Globigerinoides conglobatus at 6.6 Ma) in the South China Sea. Distinct PF faunal transition from Globorotalia siakensis– G. mayeri group to mixed-layer species occurred during the early late Miocene (∼9.6 Ma). The transition reflects a deepened upper water thermocline, possibly corresponding to the closure of the Indonesian seaway and intensification of equatorial Pacific warm currents. Abundance variations of Neogloboquadrina spp. indicate a marked shoaling of the thermocline in the northern South China Sea after 3–2.5 Ma affected possibly by a strengthened Asian winter monsoon, while a deeper thermocline existed in the southern South China Sea since the late Miocene, characterizing the west Pacific “warm pool” (WPWP). A deepening thermocline influenced by consistently warm surface water since the late Miocene in the southern South China Sea is also evidenced by a two-stepwise increase of Pulleniatina group, by a gradually decreasing Sphaeroidinella– Sphaeroidinellopsis spp. and abundant Globorotalia menardii and related species. The FO Globigerinoides ruber (pink) at approximately 1 Ma and an abrupt increase in the pink-to-all ratio of G. ruber at 0.4 Ma at Site 1143 provide two additional PF events for deciphering Pleistocene biostratigraphy and paleoceanography.

Response of upper ocean structure to the initiation of the North Hemisphere glaciation in the South China Sea

Planktonic foraminifer as well as oxygen and carbon isotopic records from Ocean Drilling Project Core 1146, located at the northern continental slope of the South China Sea, were used to study the response of the upper water column structure to the formation and progressive intensification of the Northern Hemisphere glaciation during the Late Pliocene (3.2–2.0 Ma). Variations in the relative abundance of the Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, and Globorotalia inflata groups, Globorotalia crassaformis and Neogloboquadrina pachyderma showed that sea surface temperatures gradually decreased, coinciding with heavier oxygen isotope values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma. After 2.7 Ma, the relative abundance of mixed-layer species decreased, while that of thermocline dwellers, dominated by high productivity species, increased. Δδ 13C variations among G. sacculifer, Pulleniatina obliquiloculata and Cibicidoides wuellerstorfi suggest that the mixed-layer depth and vertical exchange was enhanced with the strenghtening of the East Asian winter monsoon, which in turn is associated with the progressive intensification of the Northern Hemisphere glaciation.

Deciphering the paleoceanographic significance of Early Oligocene Braarudosphaera chalks in the South Atlantic

The recurrence of Braarudosphaera chalks in the lower Oligocene sequences of the subtropical South Atlantic has been a long-standing conundrum, with many hypotheses having been advanced to explain the genesis of these exotic nannofossil assemblages. Here, we evaluate different paleoceanographic models within the context of stable isotope (δ18O, δ13C) data measured from bulk-sediment samples and well-preserved foraminifera. Two closely-spaced Braarudosphaera layers from a lower Oligocene (foram Subzone P21a, 29.4–28.5 Ma) section drilled in the southeastern Atlantic (DSDP Site 363) are investigated. Maximum durations for the blooms that deposited the lower and upper Braarudosphaera layers are estimated to be 1.1 and 2.2 k.y., respectively. Bulk-sediment samples enriched in braarudosphaerid carbonate exhibit pronounced δ18O increases on the order of 0.6–1.0‰ which we attribute to isotopic disequilibria driven by braarudosphaerid vital effects. The two Braarudosphaera layers straddle a single peak in benthic foraminiferal δ18O values, suggesting that these blooms may recur on glacial/interglacial timescales. This same pair of braarudosphaerid layers also occurs as a couplet bundled with prolonged (∼6.7 k.y.) thermocline cooling, evidence that these stratigraphically distinct deposits may represent a ‘split signal’ for a single paleoceanographic/paleoclimatic event. Subsumed within this episode of subsurface cooling are two short-lived, negative excursions (∼0.5‰) in the δ13C record of a thermocline-dwelling planktonic foraminifer that coincide with the braarudosphaerid layers. Thus, benthic-to-thermocline δ18O and δ13C gradients were reduced during the braarudosphaerid blooms, a hallmark signature for strengthened upwelling. Both braarudosphaerid layers are marked by transient divergences in the stable isotopic signals of two shallow-dwelling species of planktonic foraminifera. These transient δ18O offsets may reflect subtle differences in the depth ecologies of these two mixed-layer species. If so, then braarudosphaerid depositional events may represent ‘subsurface blooms’ that took place within the lower parts of the euphotic zone. Alternatively, these transient δ18O offsets may reflect periods of pronounced seasonality, with braarudosphaerid blooms occurring during spring upwelling. The recurrence of Braarudosphaera blooms on both sides of the South Atlantic is believed to reflect rhythmic changes in the vigor and configuration of gyre circulation. We speculate that the termination of Braarudosphaera blooms in the South Atlantic near the end of the Early Oligocene may be related to paleoceanographic change caused by the crossing of a critical threshold in the tectonic opening of the Drake Passage and the development of the Antarctic Circum-Polar Current.

An infrared spectral reflectance study of hydrothermal alteration minerals from the Te Mihi sector of the Wairakei geothermal system, New Zealand

Hydrothermal alteration minerals and their zoning with depth in the Te Mihi sector of the Wairakei geothermal field have been characterised in this study by using field portable short-wave infrared spectroscopy. Observed variations in the spectral reflectance of hydrothermally altered volcanic and sedimentary rocks indicate a downward gradation in the dominant alteration minerals from mordenite, smectite, illite, to illite + chlorite (epidote). The variations result from temperature and permeability controlled fluid-rock interactions. Mordenite is abundant in the shallowest samples. Dioctahedral, aluminous smectites (discrete phase or as mixed-layer species) are common in the upper part of the sequence, but decrease in abundance below 200 m. In the smectite-rich zone, beidellite is common and locally may predominate over montmorillonite. Illite, including the illite component in mixed-layer illite/smectite, becomes a significant component below 150–200 m. Illite has an octahedral cation composition close to that of muscovite, with insignificant amounts of Fe and Mg. Chlorite is nearly undetectable in the upper part (0–200 m) of the altered sequence, and may become a minor component below 200 m. Chlorite is abundant only in andesitic rocks. Chlorite from the lower alteration sequence is Fe-rich, probably with the Mg# values around 0.2–0.4. Epidote is abundant locally at depth.

Tropical Pacific Ocean thermocline depth reconstructions for the Last Glacial Maximum

We evaluate the relationship between ten surface ocean (0–300 m) hydrographic parameters and the spatial distribution of factor‐analyzed core top planktonic foraminiferal abundances in the tropical Pacific Ocean (24°N–24°S) for core tops <3800 m. The spatial distribution of the first three faunal factor loadings (88% of the variance) are most highly correlated to subsurface variability (mixed layer depth, thermocline depth) and resistant species percent (RSP). However, RSP is not related to dissolution but is related to thermocline depth. Factor I (mixed layer species G. glutinata, G. ruber) and factor III (G. ruber) can be distinguished from each other by low abundances of G. glutinata in factor III. Both assemblages spatially comprise the deep mixed layer region of the western tropical and equatorial Pacific Ocean, but are associated with distinct water mass properties. A combination of Factor I and III loadings shows a higher correlation to thermocline depth (R² = 0.70). Factor II loadings (dominated by thermocline dwelling species N. dutertrei) are most significantly correlated with the thermocline depth (R² = 0.73). Most factors show only marginally significant correlation to sea surface temperatures (SSTs), indicating that SST is not the primary forcing factor on the planktonic foraminiferal species distributions in the tropical Pacific. A new transfer function was calculated to predict tropical Pacific thermocline depth from planktonic foraminifera abundances using the Imbrie‐Kipp Method (IKM) (standard deviation of residuals ±22 m (1σ)). An additional ±5‐m error is attributed to low species counts in the core top database. The modern analog technique (MAT) was also used to predict thermocline depth (standard deviation of residuals ±21 m). While last glacial maximum (LGM) thermocline depth changes by IKM and MAT were generally within error, estimated changes were geographically uniform, suggesting an oceanographic response to climate forcing. We estimate that the thermocline depth of the LGM was shallower than present by ∼20 m south of 8°S, possibly due to a shift in the South Pacific anticyclone to the northeast. Both the IKM and MAT estimate a steeper east‐west thermocline slope along the equator, suggesting that zonal wind stress (Walker circulation) was intensified during the LGM. Collectively, the thermocline estimates for the LGM suggest an equatorward compression of the climate zones in both hemispheres.

Rb-Sr Ages of Diagenesis of Mg-rich Clay in Permian Sediments, Palo Duro Basin, Texas Panhandle, U.S.A.

ABSTRACT Mid- to upper Permian sedimentary rocks in the shallow (< 3 km), intracratonic Palo Duro Basin, Texas Panhandle, occur in multicyclic carbonate-evaporite-redbed sequences, interpreted to be hypersaline shelf depositional assemblages. Certain marker horizons can be traced continuously over the entire basin. Clay minerals include distinctive varieties of Mg-rich smectite, chlorite, or mixed-layer species resulting from diagenesis in an evaporitic environment. Discrete particle-size fractions of clay were separated from core recovered from the Tubb (Leonardian), San Andres Unit #4 (lower Guadalupian), upper Seven Rivers (upper Guadalupian), and Salado-Tansill (Guadalupian-Ochoan) Formations to determine the times of mineral genesis. Rb-Sr isotopic data, for aliquots of a clay sample that has been leached by diverse ion exchange reagents, plot as a linear array in a Rb-Sr isochron diagram. For ultra-fine clay (< 0.05 µm), interpreted to be diagenetic, the slope of the data array corresponds to the time of diagenesis. Ages of clay diagenesis in the four sampled stratigraphic units agree generally with ages of deposition according to the geologic time scale, signifying that diagenesis was penecontemporaneous with deposition. Analyses of coarser clay fractions (0.2-0.5 µm and 2-4 µm) indicate the presence of a detrital component. In most instances, initial 87Sr/86Sr in diagenetic clay is higher than the Sr isotope ratio measured directly in associated anhydrite. Introduction of radiogenic Sr from occasional stream influx or from concurrent diagenetic dissolution of silicate detritus could have elevated 87Sr/86Sr in the ambient brine that mediated clay diagenesis.

Vertical thermal structure evolution of Miocene surface waters: Western equatorial Pacific DSDP Site 289

Time series analyses (from ∼ 18.7 to 5.8 Ma) of the oxygen isotopic composition of four shallow-dwelling planktic foraminifera ( Globorotalia mayeri, Dentoglobigerina altispira, Globoquadrina baroemoenensis and Globigerinoides immaturus), one deep-dwelling planktic foraminifer ( Globoquadrina venezuelana) and a benthic foraminifer ( Cibicidoides spp.) from Deep Sea Drilling Project Site 289 (2206 m present day depth) are used to determine the evolution of vertical thermal structure in the western equatorial Pacific. Comparisons of composite oxygen isotopic curves, representing the surface mixed-layer, the thermocline, and deep waters reveal three distinct intervals of stability in the position and strength of the thermocline during the Miocene (∼ 18.7 to 16.1 Ma; ∼ 11.1 to 9.9 Ma; and ∼ 7.5 to 5.8 Ma) separated by two intervals of instability of the thermocline (∼ 16.1 to 11.1 Ma; and ∼ 9.9 to 7.5 Ma). From ∼ 18.7 to 16.1 Ma, vertical thermal gradients were the weakest of the Miocene interval studied and the water column was highly stable. Vertical thermal gradients strengthened between ∼ 16.1 and 11.1 Ma as equatorial surface waters warmed and deep waters cooled during an inferred major growth phase of the East Antarctic ice sheet. As a consequence, the strength and position of the thermocline became highly variable. This was followed, from ∼ 11.1 to 9.9 Ma, by relative climatic stability, well-defined vertical thermal gradients and a slight warming of the water column. By ∼ 9.9 Ma, increasing restriction of surface water flow through the Indonesian Seaway led to the piling-up of warm surface waters in the western equatorial Pacific and a deepening of the thermocline. Deep waters continued to cool and vertical thermal gradients reached a Miocene maximum by ∼ 7.5 Ma. From ∼ 7.5 to 5.8 Ma, vertical thermal gradients were stable and the water column became highly stratified and marked by a thick, warm surface layer and a deep, well-defined thermocline. Based upon a previous assumption that ice volume growth during the middle Miocene (∼ 16.1 to 11.1 Ma) contributed to an increase inδ 18O values of 0.5 to 0.7‰ we have estimated water column temperature changes at Site 289 between ∼ 18.7 and 5.8 Ma. Surface waters are inferred to have warmed 2° to 4°C between ∼ 18.7 and 5.8 Ma, with about half of the warming occurring from ∼ 16.1 to 11.1 Ma. Between ∼ 11.1 and 9.9 Ma the entire water column warmed and from ∼ 9.9 to 7.5 Ma the mixed-layer expanded and the thermocline deepened as warm waters piled-up in the western equatorial Pacific in response to the closure of the Indonesian Seaway. The net decrease of thermoclinalδ 18O values between ∼ 11.1 and 7.5 Ma reflects an upward change in position of the deep-dwelling planktic foraminifera, Globoquadrina venezuelana, relative to the position of the thermocline as the thermocline deepened. After 7.5 Ma, relativeδ 18O values indicate that Globoquadrina venezuelana was a deep-dwelling, mixed-layer species. Deep waters (∼2400 to 2200 m) at Site 289 cooled 2° to 4°C between ∼ 18.7 and 5.8 Ma, with about half of the cooling occurring from ∼ 16.1 to 11.1 Ma and the other half from ∼ 9.9 to 7.5 Ma.

Silicate Diagenesis in the Salado Formation, Southeastern New Mexico

ABSTRACT In the Salado Formation, a thick evaporite sequence in the Permian Delaware Basin of southeastern New Mexico, primary detrital silicates such as feldspars, quartz and clay minerals have undergone significant diagenetic alteration. Seventy-five samples were collected from argillaceous partings in the lower Salado Formation halite facies, at the stratigraphic level of the Waste Isolation Pilot Plant (WIPP). These were compared with twenty-eight samples from cores of the Vaca Triste member of the Salado, a thin clastic unit at the top of the McNutt potash zone, and with a clay-rich sample from the lower contact of the Culebra Dolomite (in the overlying Rustler Formation). The purpose of this comparison was to evaluate the influence of brine chemistry (i.e., halite and potash facies, norm l to hypersaline marine conditions) and sediment composition (clays, sandy silt, dolomitized limestone) on diagenetic processes. X-ray diffraction analyses and electron microscopy identified a clay similar to corrensite from all three stratigraphic horizons. Authigenic quartz and potassium feldspar were found in both Vaca Triste samples and in those from the WIPP. The feldspar occurred in greater abundance in the Vaca Triste samples than in those from the WIPP horizon. Fluid inclusion compositions from halite in the Vaca Triste were compared with those from the WIPP repository horizon. The solution speciation/mineral solubility code EQ3NR was used to calculate ionic activities of relevant species in WIPP and Vaca Triste brines. For these high ionic strength Salado Formation brines, the activity coefficients calculated by this code must be qualified. However, the assumed pH and the computed activities of K+ /SUP> and Mg+2 correctly predict a mineralogy that is consistent with our observations of mixed-layer, Mg-rich, corrensite-like clays and microcline. EQ3NR predicts that the WIPP horizon fluid compositions are at or above saturation with respect to minerals characteristic of the halite facies: halite, aragonite, dolomite, magnesite, and huntite. Application of EQ3NR to the Vaca Triste fluid inclusions indicates saturation or supersaturation for these phases plus those typical of potash-facies mineralization (including kainite, sylvite, carnallite, syngenite, and polyhalite). We propose that the original, detrital clays reacted with magnesium-rich brines to produce a suite of Mg-rich, mixed-layer species and another brine depleted in Mg relative to K. By-products of these reactions were authigenic quartz and K-feldspar. The dissolution of plagioclase, kaolinite, and fine-grained, detrital illite probably contributed components precipitated in the authigenic phases. To summarize, a generalized reaction is suggested: brine1 + detrital silicates brine2 ± Mg-clays ± K-spar ± quartz where the K:Mg ratio in brine2 is greater than K:Mg in brine1.