Geology

doi: 10.1130/G31135.1 2010;38;867-870Geology

 Olsson, Peter J. Sugarman, Steven Tuorto and Hendra WahyudiKenneth G. Miller, Robert M. Sherrell, James V. Browning, M. Paul Field, W. Gallagher, Richard K. Cretaceous-Paleogene boundary in New JerseyRelationship between mass extinction and iridium across the  

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ABSTRACTWe directly link iridium (Ir) anomalies in New Jersey to the

mass extinction of marine plankton marking the Cretaceous-Paleo-gene (K-Pg) boundary. We confi rm previous reports of an Ir anomaly 20 cm below the extinction of Cretaceous macrofauna (the “Pinna” bed) with new results from a muddy sand section from Tighe Park, Freehold, New Jersey (United States), but we also show that Ir anom-alies correlate with marine mass extinctions at three other clay-rich New Jersey sections. Thus, we attribute the anomaly at Freehold to the downward movement of Ir and reaffi rm the link between impact and mass extinction.

INTRODUCTIONMost scientists have accepted the hypothesis that a large bolide

impact caused the Cretaceous-Paleogene (K-Pg) boundary mass extinction (Alvarez et al., 1980). A key to this debate is the detection of a large (>1 parts per billion [ppb]) Ir anomaly at the precise level of mass extinction in planktonic foraminifera and calcareous nannoplankton (Alvarez et al., 1980; Smit, 1999). The impact hypothesis is supported by identifi cation of altered impact glass and impact-derived shocked quartz (Smit, 1999; Bohor et al., 1987) and the fi nding and dating of the ~180-km-diameter Chicxulub crater (Hildebrand et al., 1991). The leading counter-hypothe-sis to impact invokes hotspot volcanism of the Deccan Traps in India (e.g., Offi cer and Drake, 1985; Courtillot et al., 1988; Keller, et al., 2008). This large igneous province consists of ~3–4 × 106 km3 of fl ood basalts (Coffi n and Eldholm, 1994). However, other studies have shown that the Deccan Trap volcanism began in early chron C29r, predating the K-Pg boundary by ~0.5 m.y., and was associated with sharp Sr-isotope and Os-isotope excursions and a global warming event (e.g., Olsson et al., 2002; Ravizza and Peucker-Ehrenbrink, 2003). Nevertheless, recent studies have re-emphasized the role of Deccan volcanism, claiming that the main pulse of volcanic outpouring occurred at the K-Pg boundary (Keller et al., 2008). This has rejuvenated the hypothesis that the mass extinction postdates the Chicxulub impact, though a recent review (Schulte et al., 2010) reaffi rms the direct relationship between a single impact and extinctions.

A key to understanding the relationship among extinctions, impact(s), and large igneous province volcanism is the location of the Ir anomaly rela-tive to the fossil record. Recent studies in New Jersey have suggested that extinction of shallow shelf Cretaceous macrofossils may have postdated the Ir anomaly (Landman et al., 2004, 2007). A 20-cm-thick fossil bed (the Pinna bed) from Tighe Park, Freehold, New Jersey (United States), is asso-ciated with the K-Pg boundary (Fig. 1); it contains a diverse latest Creta-ceous fauna including the ammonite Discoscaphites iris and bivalve Pinna laqueata (Landman et al., 2007). It is assigned to the uppermost Cretaceous Palynodinium grallator dinocyst zone. A modest Ir anomaly of ~0.5 ppb occurs at the base of the Pinna bed, suggesting two hypotheses (Landman et al., 2007): (1) the diverse Cretaceous fauna survived the impact; or (2) the Ir is displaced 20 cm down section. A third hypothesis, that the Ir is in place but the Pinna layer is a reworked tsunamite, was rejected based on the observation of P. laqueata in life position (Landman et al., 2007).

To test the relationship between Ir and the mass extinction event, we conducted a campaign of shallow coring (<25 m) at eight New Jersey localities in 2008 and 2009 adjacent to outcrops of the K-Pg boundary (Fig. 1). Data from deeper coreholes drilled onshore by Ocean Drilling Program (ODP) Legs 150X (Bass River) and 174AX (Ancora, Millville,

Geology, October 2010; v. 38; no. 10; p. 867–870; doi: 10.1130/G31135.1; 2 fi gures; Data Repository item 2010244.© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

Relationship between mass extinction and iridium across the Cretaceous-Paleogene boundary in New JerseyKenneth G. Miller1, Robert M. Sherrell1,2, James V. Browning1, M. Paul Field2, W. Gallagher3, Richard K. Olsson1, Peter J. Sugarman4, Steven Tuorto2, and Hendra Wahyudi11Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA2Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA3Department of Geological, Environmental, and Marine Sciences, Rider University, Lawrenceville, New Jersey 08468, USA4New Jersey Geological Survey, P.O. Box 427, Trenton, New Jersey 08625, USA

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Figure 1. Freehold, New Jersey, sections. Ir in parts per billion (ppb), fecal pellets in number/g of sediment, core photographs, li-thology, and formational assignment for Tighe Park 1 corehole (40°12′51.42″N, 74°17′17.79″W). First occurrence of Danian dino-cyst index fossil Damassadinium californicum is after Landman et al. (2007). Also shown is outcrop photograph (from Landman et al., 2007) from an adjacent (~190 m east) tributary (“Agony Creek”) of Manasquan River. Asterisk indicates level of Ir anomaly in this section. Inset location map shows outcrop of post-Campanian Cre-taceous (dark-green Red Bank and light-green Tinton Formations) and Paleocene (brown Hornerstown and mustard/yellow Vincentown Formations) strata; pre-Maastrichtian strata are in dark gray; post-Paleocene strata are light gray.

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and Sea Girt) also provide constraints on the relationship of extinctions to impact ejecta (Figs. 1 and 2).

METHODSIridium analyses at high spatial resolution are necessary to elucidate

the K-Pg boundary in detail. This requires Ir analysis on small samples (1 g), which we accomplished using a NiS fi re-assay technique modifi ed after Ravizza and Pyle (1997). Low procedural blanks (7 pg g–1) combined with high-sensitivity sector fi eld inductively coupled plasma–mass spec-trometry provide the detection limits (10 pg g–1 = 0.01 ppb) essential for the quantifi cation of Ir in 1 g sediment core samples. Standardization by isotope dilution yields excellent procedural reproducibility (±5%, 2σ) for even the lowest Ir concentrations (40–100 pg g–1) found in background samples. We used the resultant high-fi delity Ir data to quantify and locate the Ir anomaly with respect to other stratigraphic features associated with the K-Pg boundary.

RESULTSResults from previous drilling at downdip (~60–100 m paleodepth)

Bass River, New Jersey, show a 6-cm-thick spherule (altered microtek-tite) layer with common shocked quartz grains and carbonate accretionary lapilli (Yancey and Guillemette, 2008) that separates uppermost Creta-ceous from Paleogene sediments (Figs. 1 and 2; Olsson et al., 1997, 2002). Uppermost Cretaceous sediments underlie the spherules based on plank-

tonic foraminifera and assignment to the Palynodinium grallator dinocyst and Micula prinsii nannofossil zones. Sediments immediately above the spherule layer are assigned to lowermost Danian planktonic foraminiferal zone P0 and include the fi rst occurrence of the Danian dinofl agellate index fossil Senoniasphaera inornata (Olsson et al., 1997, 2002; Norris et al., 1999). The basal Danian contains a layer (~3 cm thick) of white clay rip-up clasts containing uppermost Cretaceous foraminifera and dinocysts. There is a modest Ir anomaly (~0.5 ppb) immediately above the spherule bed associated with the clast layer and a large Ir anomaly (2.5 ppb) at its base (Fig. 2). The global Ir anomaly is a result of stratospheric fallout and should occur above ballistic deposits, as it does at ODP Site 1049 at Blake Nose in the western North Atlantic (Norris et al., 1999), where the ejecta also occur immediately above the marine mass extinction. Thus, the Ir anomaly at the base of the spherule bed is interpreted as resulting from displacement of Ir down section 6 cm by postdepositional processes (Ols-son et al., 1997, 2002).

Other onshore New Jersey ODP sites have recovered less complete K-Pg sections, though a clay clast layer is found in most New Jersey coastal plain sites above the K-Pg boundary. Hole A at Ancora (Miller et al., 1999b) recovered white clay clasts and reworked spherules; a distinct spherule layer is absent. By contrast, Ancora Hole B contains a spherule layer, though the original microtektites have been redeposited. Although biostratigraphically complete, the ODP Leg 174AX Site Millville (Sug-arman et al., 2005) K-Pg boundary lacks impact spherules and shocked

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Figure 2. Other subsurface sections. Ir in parts per billion (ppb), fecal pellets in number/g of sediment, core photographs, lithology, and formational assignment for Buck Pit 1 (40°14′18.79″N, 74°24′45.85″W), Search Farm 1 (40°05′29.20″N, 74°32′16.10″W), and Meirs Farm 1 (40°06′15.48″N, 74°31′37.48″W) coreholes. Note that scale for Ir at Bass River (Olsson et al., 1997) is greatly compressed relative to other locations. Lowest occurrence of Senoniasphaera inornata is at base of green clay in adjacent Buck Pit outcrop (E. Rudolph, 1994, personal commun.). K-Pg—Cretaceous-Paleogene.

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minerals. ODP Leg 174AX Site Sea Girt, spot cores from Parvin, New Jersey, and outcrops at Sewell, New Jersey (Fig. 1), recovered clay clasts but no spherules (Miller et al., 1999b, 2006). These fi ndings refl ect the heterogeneity of preservation of spherules on the New Jersey coastal plain. The pervasive white clay clasts are interpreted as rip-ups result-ing from a tsunami that eroded the outer continental shelf and upper slope (Olsson et al., 1997, 2002; Norris et al., 2000). The clasts contain Maastrichtian planktonic foraminifera and a diverse outer neritic benthic foraminiferal assemblage; this contrasts with inner neritic environments in the enveloping Hornerstown Formation, supporting tsunami erosion of the outer shelf. This tsunami was due to slope failure triggered by earthquakes generated by the Chicxulub impact (Norris et al., 2000; Ols-son et al., 2002) and is distinct from the impact-generated mega-tsunami that reworked sediments in the Gulf of Mexico but was blocked from the open Atlantic (Norris et al., 2000).

We compare our new outcrop and corehole studies with previous results from outcrop and the Bass River corehole (Figs. 1 and 2). The Tighe Park 1 corehole is adjacent to (~190 m) the Agony Creek type sec-tion of the Pinna bed and faithfully reproduces the stratigraphy in the out-crop (Fig. 1; Table DR1 in the GSA Data Repository1). Meirs Farm 1, Buck Pit 1, and Search Farm 1 cores (Fig. 2) provide a slightly different physical stratigraphy, with generally fi ner-grained sediments than Tighe Park I (Table DR1). Local lithologic variations are complex spanning the boundary: a basal Danian burrowed glauconite sand of the Hornerstown Formation overlies various lithologies, including clayey glauconite sands and glauconitic clays (Navesink and New Egypt Formations) and mostly indurated quartz sands (Tinton Formation)(Table DR1). Biostratigraphic control on the shallow coreholes is limited because of the absence of cal-careous microfossils, though dinocyst and macrofossil data allow confi -dent placement of the K-Pg boundary (Table DR1). Downdip cores from Bass River and Ancora allow fi rm identifi cation of the K-Pg boundary using calcareous and dinocyst microfossils (Fig. 2). The contrasting litho-facies spanning the K-Pg boundary at these sites allows us to test the rela-tionships among Ir, lithology, and impact-related features.

We measured Ir and compared it to the basic stratigraphy outlined earlier herein and to counts of echinoid fecal pellets. The appearance of epifaunal echinoid fecal pellets is a distinct lowermost Danian strati-graphic horizon that is used to correlate Tighe Park outcrops to Meirs Farm 1, Search Farm 1, Buck Pit 1, and Bass River coreholes (Fig. 2). We suggest that the dramatic decrease in export production following the extinction event (D’Hondt, 2005) resulted in increased burrowing and benthic bulldozing.

The Tighe Park 1 corehole (Fig. 1) shows an Ir anomaly below the sandy Pinna bed (i.e., it occurs in uppermost Cretaceous sediments), confi rming the relationship observed between Ir and the Pinna bed in the adjacent outcrop (Landman, et al., 2007) and apparently predating the extinctions. At Meirs Farm 1, the Ir anomaly occurs in clay-rich sedi-ments containing a white clay clast layer correlated to the interval above the extinctions at Bass River (Fig. 2). At Buck Pit 1, the Ir anomaly also occurs in clay-rich sediments and is associated with the lowest occur-rence of Senoniasphaera inornata, a marker for the base of the Danian. At Search Farm 1, the anomaly occurs just above a Cucullaea vulgaris layer, an abundant species in the Pinna bed at Tighe Park, and is associ-ated with an increase in fecal pellets. Thus, Meirs Farm 1, Buck Pit 1, and Search Farm 1 show that the Ir anomaly is basal Danian and that it immediately postdates the extinctions. At Bass River, the Ir anomaly is at the base of the spherule layer, 8 cm below the white clay clast layer,

but it clearly still postdates the extinctions of Cretaceous planktonic foraminifera.

Comparisons of these sections show that in clay-rich sections, the Ir anomaly correlates with the extinctions, but in sandier sections, the Ir is found 6 cm (Bass River) to 20 cm (Tighe Park) lower. The Ir anomaly at Tighe Park 1 is a sharp peak at a redox boundary that occurs 20 cm below the appearance of Danian dinocysts; this is similar to Bass River, where the anomaly occurs 6 cm below the appearance of Danian foraminifera and dinocysts. This differs from the other three sites, where broader Ir peaks are associated with the appearance of Danian dinocysts (Buck Pit 1) or the up-section increase in fecal pellets (Search Farm 1, Meirs Farm 1) that correlates with the appearance of Danian foraminifera and dinocysts.

Originally considered relatively immobile, postdepositional mobi-lization/focusing of Ir and other platinum group elements is now well established. Nonimpact Ir (terrestrial) associated with authigenic Fe-Mn can be reduced, solubilized, mobilized (20 cm), and deposited at redox boundaries (Colodner et al., 1992). Modest Ir anomalies at the Devonian-Carboniferous boundary are attributed to focusing of terrestrial Ir by redox conditions during deposition and/or early diagenesis (Wang et al., 1993). Although Ir may be mobilized and redeposited up to 1 m from its origi-nal location, evidence for an extraterrestrial source can be provided by chondritic Ir/Pt ratios associated with the K-Pg Ir anomaly (Norris et al., 2000; Robinson et al., 2009). As an example of this, Ir at Bass River was mobilized after initial deposition at the top of the permeable spherule layer and concentrated at the base of the spherule layer at an aquiclude. We suggest that this explanation similarly applies to the type section of the Pinna bed at Tighe Park, where the Ir was mobilized, percolated through the sandy Pinna bed, and accumulated in a 2-cm-thick zone at a distinct contact on top of the indurated Tinton Formation, also an aquiclude. The mobilization and redeposition of Ir were likely due to differences in redox potential (Colodner et al., 1992). Ir is likely mobilized up and down sec-tion, but accumulates only where the sediments become more oxidizing (e.g., at the base of the spherule layer at Bass River and Pinna bed at Tighe Park 1). Thus, our studies strongly suggest that the Ir has been mobilized at Tighe Park and Bass River, weakening the case for the hypothesis that the extinctions postdate impact. We cannot refute the alternate hypothesis that the Pinna bed could be a unique survivor layer. However, considering that the Ir anomaly at Tighe Park/Agony Creek is displaced, as it is at Bass River, there is no evidence to support this hypothesis. We suggest that the mobility of Ir requires caution when using the Ir anomaly alone as a pre-cise means of global correlation.

The Ir anomaly at Tighe Park, Buck Pit 1, Search Farm 1, and Meirs Farms 1 is signifi cantly lower (~0.5 ppb) than at Bass River (~2.5 ppb) or other locations (Smit, 1999). Because the input of Ir was due to rapid atmospheric fallout, the lower values observed are not due to dilution by sedimentation rate differences, but must be due to bioturbation, vertical mixing, or pore-water mobility of the signal. Baseline Ir concentrations tend to have different values below and above the anomaly, suggesting dif-ferential up/down mobility of impact Ir over approximately a meter scale.

CONCLUSIONThe end Cretaceous mass extinction was predated by a large cool-

ing and sea-level fall at the Campanian-Maastrichtian boundary (71.5 Ma) (e.g., Miller et al., 1999a), a latest Cretaceous warming ~0.5 m.y. before the boundary, a minor cooling just prior to (tens of thousands of years) the extinction (Olsson et al., 2002), and massive volcanism associated with the Deccan Traps (e.g., Keller, et al., 2008). Sea level fell slowly during the Maastrichtian, with a gradual fall across the boundary and a major fall ~0.5 m.y. after the boundary. These tumultuous Maastrichtian climatic events undoubtedly contributed to a decline in species diversity prior to the K-Pg boundary, but the evidence is clear: impact-related spherules and an Ir anomaly are associated with the mass extinction of the marine plankton

1GSA Data Repository item 2010244, Table DR1 (lithologies and age in-formation from the boreholes discussed in the text), is available online at www.geosociety.org/pubs/ft2010.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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870 GEOLOGY, October 2010

(Fig. 2) and, by extension, the shallow-water invertebrates, marine verte-brates, and terrestrial vertebrates.

ACKNOWLEDGMENTSWe thank G. Ravizza for advice about Ir extraction, E. Boyle for the enriched

Ir spike, E. Rudolph for dinocyst data, S. Esmeray for discussions, R. Norris and two anonymous reviewers for comments, the New Jersey Geological Survey, the U.S. Geological Survey Eastern Region Mapping Team drillers for collecting the cores, L. Campo, D. Meirs, W. Search, W. Stone, and the staff of Tighe Park for access, and funding by National Science Foundation grant EAR-070778. We thank N. Landman and R. Johnson for comments.

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Manuscript received 19 February 2010Revised manuscript received 4 May 2010Manuscript accepted 6 May 2010

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